Menu
Home
People
Places
Arts
History
Plants & Animals
Science
Life & Culture
Technology
Reference.org
Fertility
open-in-new
<h2 id="demography">Demography</h2> <p>In demographic contexts, fertility refers to the actual production of offspring, rather than the physical capability to produce which is termed <a href="/facts/Fecundity/e0oMColj">fecundity</a>.<a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a><a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a> While fertility can be measured, fecundity cannot be. Demographers measure the fertility rate in a variety of ways, which can be broadly broken into "period" measures and "<a href="/facts/Cohort_(statistics)/6uqoVNft">cohort</a>" measures. "Period" measures refer to a cross-section of the population in one year. "Cohort" data on the other hand, follows the same people over a period of decades. Both period and cohort measures are widely used.<a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a> </p> <h3>Period measures</h3> <ul><li><a href="/facts/Crude_birth_rate/t7GJHp2x">Crude birth rate</a> (CBR) - the number of <a href="/facts/Live_birth_(human)/6U8SFb5l">live births</a> in a given year per 1,000 people alive at the middle of that year. One disadvantage of this indicator is that it is influenced by the age structure of the population.</li> <li>General fertility rate (GFR) - the number of births in a year divided by the number of women aged 15–44, times 1000. It focuses on the potential mothers only, and takes the age distribution into account.</li> <li>Child-Woman Ratio (CWR) - the ratio of the number of children under 5 to the number of women 15–49, times 1000. It is especially useful in historical data as it does not require counting births. This measure is actually a hybrid, because it involves deaths as well as births. (That is, because of <a href="/facts/Infant_mortality/zlUfrFPK">infant mortality</a> some of the births are not included; and because of adult mortality, some of the women who gave birth are not counted either.)</li> <li>Coale's Index of Fertility - a special device used in historical research</li></ul> <h3>Cohort measures</h3> <ul><li><a href="/facts/Total_fertility_rate/ugganzwn">Total fertility rate</a> (TFR) - the total number of children a woman would bear during her lifetime if she were to experience the prevailing age-specific fertility rates of women. TFR equals the sum for all age groups of 5 times each ASFR rate.<a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a></li> <li>Gross Reproduction Rate (GRR) - the number of girl babies a synthetic cohort will have. It assumes that all of the baby girls will grow up and live to at least age 50.</li> <li>Net Reproduction Rate (NRR) - the NRR starts with the GRR and adds the realistic assumption that some of the women will die before age 49; therefore they will not be alive to bear some of the potential babies that were counted in the GRR. NRR is always lower than GRR, but in countries where mortality is very low, almost all the baby girls grow up to be potential mothers, and the NRR is practically the same as GRR. In countries with high mortality, NRR can be as low as 70% of GRR. When NRR = 1.0, each generation of 1000 baby girls grows up and gives birth to exactly 1000 girls. When NRR is less than one, each generation is smaller than the previous one. When NRR is greater than 1 each generation is larger than the one before. NRR is a measure of the long-term future potential for growth, but it usually is different from the current population growth rate.</li></ul> <h3>Social and economic determinants of fertility</h3> <p class="note">Main article: <a href="/facts/Fertility_factor_(demography)/rd2XqhFB">fertility factor (demography)</a></p> <p>A parent's number of children strongly correlates with the number of children that each person in the next generation will eventually have.<a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a> Factors generally associated with increased fertility include <a href="/facts/Religiosity/Gc19pSN0">religiosity</a>,<a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a> intention to have children,<a class="footnote-ref" id="fnref:13" href="#fn:13"><sup>13</sup></a> and maternal support.<a class="footnote-ref" id="fnref:14" href="#fn:14"><sup>14</sup></a> Factors generally associated with decreased fertility include <a href="/facts/Income_and_fertility/hGkUqLjY">wealth</a>, education,<a class="footnote-ref" id="fnref:15" href="#fn:15"><sup>15</sup></a><a class="footnote-ref" id="fnref:16" href="#fn:16"><sup>16</sup></a> <a href="/facts/Female_labor_participation/72oAHLyE">female labor participation</a>,<a class="footnote-ref" id="fnref:17" href="#fn:17"><sup>17</sup></a> <a href="/facts/Urban_area/fu9v8h5P">urban</a> residence,<a class="footnote-ref" id="fnref:18" href="#fn:18"><sup>18</sup></a> <a href="/facts/Affordable_housing/4q4bTrSu">cost of housing</a>,<a class="footnote-ref" id="fnref:19" href="#fn:19"><sup>19</sup></a><a class="footnote-ref" id="fnref:20" href="#fn:20"><sup>20</sup></a> <a href="/facts/Intelligence_and_fertility/3HnVetzA">intelligence</a>, <a href="/facts/Age_and_female_fertility/S2dACXng">increased female age</a> and (to a lesser degree) <a href="/facts/Paternal_age_effect/hXt4hI9d">increased male age</a>. </p><p>The "Three-step Analysis" of the fertility process was introduced by Kingsley Davis and Judith Blake in 1956 and makes use of three proximate determinants:<a class="footnote-ref" id="fnref:21" href="#fn:21"><sup>21</sup></a><a class="footnote-ref" id="fnref:22" href="#fn:22"><sup>22</sup></a> The economic analysis of fertility is part of <a href="/facts/Household_economics/dmWL5Lsp">household economics</a>, a field that has grown out of the <a href="/facts/New_Home_Economics/Kzg50VhI">New Home Economics</a>. Influential economic analyses of fertility include Becker (1960),<a class="footnote-ref" id="fnref:23" href="#fn:23"><sup>23</sup></a> Mincer (1963),<a class="footnote-ref" id="fnref:24" href="#fn:24"><sup>24</sup></a> and <a href="/facts/Richard_Easterlin/F7SdN5E5">Easterlin</a> (1969).<a class="footnote-ref" id="fnref:25" href="#fn:25"><sup>25</sup></a> The latter developed the <a href="/facts/Easterlin_hypothesis/6dzIF1DU">Easterlin hypothesis</a> to account for the Baby Boom. </p> <h3>Bongaarts' model of components of fertility</h3> <p><a href="/facts/John_Bongaarts/sbsnFNdD">Bongaarts</a> proposed a model where the total fertility rate of a population can be calculated from four proximate determinants and the total fecundity (TF). The index of marriage (Cm), the index of contraception (Cc), the index of induced abortion (Ca) and the index of postpartum infecundability (Ci). These indices range from 0 to 1. The higher the index, the higher it will make the TFR, for example a population where there are no induced abortions would have a Ca of 1, but a country where everybody used infallible contraception would have a Cc of 0. </p><p>TFR = TF × Cm × Ci × Ca × Cc </p><p>These four indices can also be used to calculate the total marital fertility (TMFR) and the total natural fertility (TN). </p><p>TFR = TMFR × Cm </p><p>TMFR = TN × Cc × Ca </p><p>TN = TF × Ci </p> Intercourse The first step is <a href="/facts/Sexual_intercourse/okjwpVnB">sexual intercourse</a>, and an examination of the average age at first intercourse, the average frequency outside marriage, and the average frequency inside. <a href="/facts/Fertilisation/g8u3UKzF">Conception</a> Certain physical conditions may make it impossible for a woman to conceive. This is called "involuntary infecundity." If the woman has a condition making it possible, but unlikely to conceive, this is termed "subfecundity." <a href="/facts/Venereal_diseases/IYL1rxdI">Venereal diseases</a> (especially <a href="/facts/Gonorrhea/xulQCkLF">gonorrhea</a>, <a href="/facts/Syphilis/fdnGLyLT">syphilis</a>, and <a href="/facts/Chlamydia_infection/bcKTIM1L">chlamydia</a>) are common causes. <a href="/facts/Nutrition/SMJ3Dhfs">Nutrition</a> is a factor as well: women with less than 20% body fat may be subfecund, a factor of concern for <a href="/facts/Sportsperson/DDwPug4A">athletes</a> and people susceptible to <a href="/facts/Anorexia_nervosa/00KSqEn7">anorexia</a>. Demographer Ruth Frisch has argued that "It takes 50,000 calories to make a baby". There is also subfecundity in the weeks following childbirth, and this can be prolonged for a year or more through <a href="/facts/Breastfeeding/cdWXyYDX">breastfeeding</a>. A furious political debate raged in the 1980s over the ethics of baby food companies marketing infant formula in developing countries. A large industry has developed to <a href="/facts/Infertility_treatment/f3qpbE3X">deal with subfecundity</a> in women and men. An equally large industry has emerged to provide <a href="/facts/Contraception/qCzhxgH8">contraceptive</a> devices designed to prevent conception. Their effectiveness in use varies. On average, 85% of married couples using no contraception will have a <a href="/facts/Pregnancy/vEKYGpep">pregnancy</a> in one year. The rate drops to the 20% range when using withdrawal, vaginal sponges, or spermicides. (This assumes the partners never forget to use the contraceptive.) The rate drops to only 2 or 3% when using <a href="/facts/The_pill/Wudd4dH7">the pill</a> or an <a href="/facts/IUD/DSbzljJo">IUD</a>, and drops to near 0% for implants and 0% for <a href="/facts/Tubal_ligation/7AHjL2h2">tubal ligation</a> (<a href="/facts/Sterilization_(medicine)/q2YYTfB1">sterilization</a>) of the woman, or a <a href="/facts/Vasectomy/TV9CDGSz">vasectomy</a> for the man. <a href="/facts/Gestation/QKgkykw5">Gestation</a> After a <a href="/facts/Fetus/evocHd0Y">fetus</a> is conceived, it may or may not survive to birth. "Involuntary fetal mortality" involves natural abortion, <a href="/facts/Miscarriage/sFq0vdnU">miscarriages</a> and <a href="/facts/Stillbirth/x3UcW2P7">stillbirth</a> (a fetus born dead). Human intervention intentionally causing abortion of the fetus is called "<a href="/facts/Therapeutic_abortion/ro0RjIBT">therapeutic abortion</a>". <h2 id="in-medicine">In medicine</h2> <p>In medicine, the definition of fertility is "the capacity to establish a clinical pregnancy."<a class="footnote-ref" id="fnref:26" href="#fn:26"><sup>26</sup></a> </p><p>Women have <a href="/facts/Hormonal/WFnBnINJ">hormonal</a> cycles which determine when they can achieve <a href="/facts/Pregnancy/vEKYGpep">pregnancy</a>. The cycle is approximately twenty-eight days long, with a fertile period of five days per cycle, but can deviate greatly from this norm. Men are fertile continuously, but their <a href="/facts/Sperm_quality/DgsKJmyp">sperm quality</a> is affected by their health, frequency of ejaculation, and environmental factors.<a class="footnote-ref" id="fnref:27" href="#fn:27"><sup>27</sup></a> </p><p>Fertility declines with age in both sexes. For women, the decline begins around the age of 32, and becomes precipitous at age 37. For men, potency and sperm quality begins to decline around the age of 40. Even if an older couple does manage to conceive a child, the pregnancy will be increasingly difficult for the mother, and carries a higher risk of <a href="/facts/Birth_defect/ESfzQhTx">birth defects</a> and <a href="/facts/Genetic_disorder/hFqt2NLn">genetic disorders</a> for the child.<a class="footnote-ref" id="fnref:28" href="#fn:28"><sup>28</sup></a> </p><p><a href="/facts/Pregnancy_rate/hEDdgYMo">Pregnancy rates for sexual intercourse</a> are highest when it occurs every 1 or 2 days,<a class="footnote-ref" id="fnref:29" href="#fn:29"><sup>29</sup></a> or every 2 or 3 days.<a class="footnote-ref" id="fnref:30" href="#fn:30"><sup>30</sup></a> Studies have found no significant difference between different <a href="/facts/Sex_positions/hGbEF6hH">sex positions</a> and pregnancy rate, as long as it results in <a href="/facts/Ejaculation/5RnfwCxG">ejaculation</a> into the <a href="/facts/Vagina/ca5jsTwF">vagina</a>.<a class="footnote-ref" id="fnref:31" href="#fn:31"><sup>31</sup></a> </p> <h3>Menstrual cycle</h3> <p class="note">Main article: <a href="/facts/Menstrual_cycle/AY6Y9xxM">Menstrual cycle</a></p> <p>A woman's menstrual cycle begins, as arbitrarily assigned, with menses. Next is the follicular phase where estrogen levels build as an ovum matures (due to the follicular stimulating hormone, or FSH) within the ovary. When estrogen levels peak, it spurs a surge of luteinizing hormone (LH) which completes maturation and enables the ovum to break through the ovary wall.<a class="footnote-ref" id="fnref:32" href="#fn:32"><sup>32</sup></a> This is ovulation. During the luteal phase following ovulation LH and FSH cause the post-ovulation ovary to develop into the corpus luteum which produces progesterone. The production of progesterone inhibits the LH and FSH hormones which (in a cycle without pregnancy) causes the corpus luteum to atrophy, and menses to begin the cycle again. </p><p>Peak fertility occurs during just a few days of the cycle: usually two days before and two days after the ovulation date.<a class="footnote-ref" id="fnref:33" href="#fn:33"><sup>33</sup></a> This <a href="/facts/Fertile_window/AY6Y9xxM">fertile window</a> varies from woman to woman, just as the ovulation date often varies from cycle to cycle for the same woman.<a class="footnote-ref" id="fnref:34" href="#fn:34"><sup>34</sup></a> The <a href="/facts/Ovum/gJyW3W3W">ovule</a> is usually capable of being fertilized for up to 48 hours after it is released from the <a href="/facts/Ovary/z2hG6bNy">ovary</a>. <a href="/facts/Sperm/RCxpXygQ">Sperm</a> survive inside the <a href="/facts/Uterus/LIrApMXs">uterus</a> between 48 and 72 hours on average, with the maximum being 120 hours (5 days). </p><p>These periods and intervals are important factors for couples using the <a href="/facts/Rhythm_method/Go1Aj7xn">rhythm method</a> of contraception. </p> <h3>Female fertility</h3> <p class="note">Further information: <a href="/facts/Female_infertility/gWyt2796">Female infertility</a></p> <p>The average age of <a href="/facts/Menarche/EFzGSWMk">menarche</a> in the United States is about 12.5 years.<a class="footnote-ref" id="fnref:35" href="#fn:35"><sup>35</sup></a> In postmenarchal girls, about 80% of the cycles are <a href="/facts/Anovulation/nbg5Speh">anovulatory</a> (ovulation does not actually take place) in the first year after menarche, 50% in the third and 10% in the sixth year.<a class="footnote-ref" id="fnref:36" href="#fn:36"><sup>36</sup></a> </p><p><a href="/facts/Menopause/b3Fz6NHb">Menopause</a> occurs during a woman's midlife between ages 48 and 55.<a class="footnote-ref" id="fnref:37" href="#fn:37"><sup>37</sup></a><a class="footnote-ref" id="fnref:38" href="#fn:38"><sup>38</sup></a> During menopause, hormonal production by the ovaries is reduced, eventually causing a permanent cessation of the creation of the uterine lining (period). This is considered the end of the fertile phase of a woman's life. </p><p>The predicted effect of <a href="/facts/Age_and_female_fertility/S2dACXng">age on female fertility</a> in women trying to get pregnant, without using fertility drugs or in vitro fertilization:<a class="footnote-ref" id="fnref:39" href="#fn:39"><sup>39</sup></a> </p> <ul><li>At age 30 <ul><li>75% will conceive ending in a live birth within one year</li> <li>91% will conceive ending in a live birth within four years.</li></ul></li> <li>At age 35 <ul><li>66% will conceive ending in a live birth within one year</li> <li>84% will conceive ending in a live birth within four years.</li></ul></li> <li>At age 40 <ul><li>44% will conceive ending in a live birth within one year</li> <li>64% will conceive ending in a live birth within four years.</li></ul></li></ul> <p><a class="footnote-ref" id="fnref:40" href="#fn:40"><sup>40</sup></a> </p><p>Studies of couples trying to conceive have yielded better results: one 2004 study of 770 European women found that 82% of 35- to 39-year-old women conceived within a year,<a class="footnote-ref" id="fnref:41" href="#fn:41"><sup>41</sup></a> while a study in 2013 of 2,820 Danish women saw 78% of 35- to 40-year-olds conceive within a year.<a class="footnote-ref" id="fnref:42" href="#fn:42"><sup>42</sup></a> </p><p>According to an opinion by the Practice Committee of the American Society for Reproductive Medicine, specific coital timing or position, and resting supine after intercourse have no significant impact on fertility. Sperm can be found in the cervical canal seconds after ejaculation, regardless of coital position.<a class="footnote-ref" id="fnref:43" href="#fn:43"><sup>43</sup></a> </p><p>Successful pregnancies facilitated by fertility treatment have been documented in <a href="/facts/Pregnancy_over_age_50/elkttuEQ">women as old as 67</a>.<a class="footnote-ref" id="fnref:44" href="#fn:44"><sup>44</sup></a> </p> <h3>Male fertility</h3> <p class="note">See also: <a href="/facts/Paternal_age_effect/hXt4hI9d">Paternal age effect</a> and <a href="/facts/Male_infertility/wrF5RGjs">Male infertility</a></p> <p>Some research suggests that older males have decreased semen volume, <a href="/facts/Sperm_motility/PKHcbdjk">sperm motility</a>, and impaired sperm morphology.<a class="footnote-ref" id="fnref:45" href="#fn:45"><sup>45</sup></a> In studies that controlled for female partner's age, comparisons between men under 30 and men over 50 found relative decreases in <a href="/facts/Pregnancy_rate/hEDdgYMo">pregnancy rates</a> between 23% and 38%.<a class="footnote-ref" id="fnref:46" href="#fn:46"><sup>46</sup></a> <a href="/facts/Sperm_count/orD9znfB">Sperm count</a> declines with age, with men aged 50–80 years producing sperm at an average rate of 75% compared with men aged 20–50 years and larger differences exist in the number of <a href="/facts/Seminiferous_tubules/Jytp8phd">seminiferous tubules</a> in the <a href="/facts/Testes/Doy40hoq">testes</a> containing mature sperm:<a class="footnote-ref" id="fnref:47" href="#fn:47"><sup>47</sup></a> </p> <ul><li>In males 20–39 years old, 90% of the seminiferous tubules contain mature sperm.</li> <li>In males 40–69 years old, 50% of the seminiferous tubules contain mature sperm.</li> <li>In males 80 years old and older, 10% of the seminiferous tubules contain mature sperm.<a class="footnote-ref" id="fnref:48" href="#fn:48"><sup>48</sup></a></li></ul> <p>Decline in male fertility is influenced by many factors, including lifestyle, environment and psychological factors.<a class="footnote-ref" id="fnref:49" href="#fn:49"><sup>49</sup></a> </p><p>Some research suggests increased risks for health problems for children of older fathers, but no clear association has been proven.<a class="footnote-ref" id="fnref:50" href="#fn:50"><sup>50</sup></a> A large scale study in <a href="/facts/Israel/dEe3IcaY">Israel</a> suggested that the children of men 40 or older were 5.75 times more likely than children of men under 30 to have an <a href="/facts/Autism_spectrum_disorder/bzouRyMI">autism spectrum disorder</a>, controlling for year of birth, socioeconomic status, and maternal age.<a class="footnote-ref" id="fnref:51" href="#fn:51"><sup>51</sup></a> <a href="/facts/Paternal_age_effect/hXt4hI9d">Increased paternal age</a> has been suggested to correlate with <a href="/facts/Schizophrenia/FMP4lQ57">schizophrenia</a> but it is unproven.<a class="footnote-ref" id="fnref:52" href="#fn:52"><sup>52</sup></a><a class="footnote-ref" id="fnref:53" href="#fn:53"><sup>53</sup></a><a class="footnote-ref" id="fnref:54" href="#fn:54"><sup>54</sup></a><a class="footnote-ref" id="fnref:55" href="#fn:55"><sup>55</sup></a><a class="footnote-ref" id="fnref:56" href="#fn:56"><sup>56</sup></a> </p><p>Australian researchers have found evidence to suggest <a href="/facts/Obesity_and_fertility/59zjoU8l">obesity</a> may cause subtle damage to sperm and prevent a healthy pregnancy. They reported fertilization was 40% less successful when the father was overweight.<a class="footnote-ref" id="fnref:57" href="#fn:57"><sup>57</sup></a> </p><p>The <a href="/facts/American_Fertility_Society/wtqaNy7S">American Fertility Society</a> recommends an age limit for sperm donors of 50 years or less,<a class="footnote-ref" id="fnref:58" href="#fn:58"><sup>58</sup></a> and many <a href="/facts/Fertility_clinic/5YP41rjP">fertility clinics</a> in the <a href="/facts/United_Kingdom/kuPAILt2">United Kingdom</a> will not accept donations from men over 40 or 45 years of age.<a class="footnote-ref" id="fnref:59" href="#fn:59"><sup>59</sup></a> </p> <h2 id="historical-trends-by-country">Historical trends by country</h2> <p class="note">Main article: <a href="/facts/Historical_demography/nzwMqY4R">Historical demography</a></p> <p class="note">See also: <a href="/facts/Demographic_transition/KHTKXRjS">Demographic transition</a></p> <h3>France</h3> <p class="note">Main article: <a href="/facts/Demographics_of_France/eo5l1fHh">Demographics of France</a></p> <p>The <a href="/facts/France/CxIeIKM3">French</a> <a href="/facts/Pronatalist/72wAxaLP">pronatalist</a> movement from 1919 to 1945 failed to convince French couples they had a <a href="/facts/Patriotism/NTmYkdwf">patriotic</a> duty to help increase their country's birthrate. Even the government was reluctant to support the movement. Only between 1938 and 1939 did the French government become directly and permanently involved in the pronatalist effort. Although the birthrate started to surge in late 1941, the trend was not sustained. Falling birthrate again became a major concern among demographers and government officials in the 1970s.<a class="footnote-ref" id="fnref:60" href="#fn:60"><sup>60</sup></a> In mid-2018, there was a bill introduced to legalize single women and lesbian couples to get fertility treatment. At the beginning of 2020, the Senate approved the bill 160 votes to 116. They are a step closer to legalizing <a href="/facts/Fertility_treatment/f3qpbE3X">fertility treatments</a> for all women regardless of sexual orientation or marital status. Soon there will be no reason for lesbian couples or single women to travel to be able to start their own family.<a class="footnote-ref" id="fnref:61" href="#fn:61"><sup>61</sup></a> </p> <h3>Korea</h3> <p class="note">Main article: <a href="/facts/Low_birth_rate_in_South_Korea/YwQfcLE3">Low birth rate in South Korea</a></p> <p><a href="/facts/South_Korea/pCobAaVw">South Korea</a> has the lowest fertility rate in the world at 0.78.<a class="footnote-ref" id="fnref:62" href="#fn:62"><sup>62</sup></a> A variety of explanations have been proposed, ranging from investment in education<a class="footnote-ref" id="fnref:63" href="#fn:63"><sup>63</sup></a> to <a href="/facts/Birth_control/qCzhxgH8">birth control</a>, <a href="/facts/Abortion/ro0RjIBT">abortion</a>, a decline in the marriage rate, <a href="/facts/Divorce/tENy6fK3">divorce</a>, female participation in the labor force, and the <a href="/facts/1997_Asian_financial_crisis/EfxsMxXQ">1997 Asian financial crisis</a>.<a class="footnote-ref" id="fnref:64" href="#fn:64"><sup>64</sup></a> After being legal from the 1960s to the 1980s, abortion was again made illegal in South Korea in the early 2000s in an attempt to reverse the declining fertility rate.<a class="footnote-ref" id="fnref:65" href="#fn:65"><sup>65</sup></a> </p> <h3>United States</h3> <p class="note">Main article: <a href="/facts/Demographic_history_of_the_United_States/tq3FFyAP">Demographic history of the United States</a></p> <p>From 1800 to 1940, fertility fell in the US. There was a marked decline in fertility in the early 1900s, associated with improved contraceptives, <a href="/facts/Birth_control_movement_in_the_United_States/ts3hfJ4P">greater access to contraceptives and sexuality information</a> and the "first" <a href="/facts/Sexual_revolution/bvgA4CWk">sexual revolution</a> in the 1920s. </p> <h4>Post-WWII</h4> <p class="note">Main article: <a href="/facts/Baby_Boom/wtsEMNMe">Baby Boom</a></p> <p>After 1940 fertility suddenly started going up again, reaching a new peak in 1957. After 1960, fertility started declining rapidly. In the Baby Boom years (1946–1964), women married earlier and had their babies sooner; the number of children born to mothers after age 35 did not increase.<a class="footnote-ref" id="fnref:66" href="#fn:66"><sup>66</sup></a> </p> <h4>Sexual revolution</h4> <p class="note">See also: <a href="/facts/Sexual_revolution_in_1960s_America/l6c42WMs">Sexual revolution in 1960s America</a></p> <p>After 1960, new methods of contraception became available, the ideal family size fell, from 3 to 2 children. Couples postponed marriage and first births, and they sharply reduced the number of third and fourth births.<a class="footnote-ref" id="fnref:67" href="#fn:67"><sup>67</sup></a> </p> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Anti-natalism/lFMsTLM8">Anti-natalism</a></li> <li><a href="/facts/Birth_control/qCzhxgH8">Birth control</a></li> <li><a href="/facts/Family_economics/vSk5vcAc">Family economics</a></li> <li><a href="/facts/Family_planning/SwCbJ8Yd">Family planning</a></li> <li><a href="/facts/Fecundity/e0oMColj">Fecundity</a></li> <li><a href="/facts/Fertility_clinic/5YP41rjP">Fertility clinic</a></li> <li><a href="/facts/Fertility_tourism/oNlnaYAM">Fertility tourism</a></li> <li><a href="/facts/Fertility_deity/e8xMqyd9">Fertility deity</a></li> <li><a href="/facts/Fertility_preservation/NrjReGYw">Fertility preservation</a></li> <li><a href="/facts/Human_Fertilisation_and_Embryology_Authority/J4s7bJLM">Human Fertilisation and Embryology Authority</a></li> <li><a href="/facts/Natalism/72wAxaLP">Natalism</a></li> <li><a href="/facts/Natural_fertility/oHCpReCt">Natural fertility</a></li> <li><a href="/facts/Oncofertility/a7gfbkNE">Oncofertility</a></li> <li><a href="/facts/Reproductive_health/aQ4gq2xp">Reproductive health</a></li> <li><a href="/facts/Sub-replacement_fertility/AG7SSeDT">Sub-replacement fertility</a></li> <li><a href="/facts/Total_fertility_rate/ugganzwn">Total fertility rate</a></li> <li><a href="/facts/Vasectomy/TV9CDGSz">Vasectomy</a></li> <li><a href="/facts/Fertility-development_controversy/hGkUqLjY">Fertility-development controversy</a></li> <li><a href="/facts/Fertility_factor_(demography)/rd2XqhFB">Fertility factor (demography)</a> <ul><li><a href="/facts/Income_and_fertility/hGkUqLjY">Income and fertility</a></li> <li><a href="/facts/Fertility_and_intelligence/3HnVetzA">Fertility and intelligence</a></li></ul></li></ul> <h2 id="further-reading">Further reading</h2> <ul><li>Bloom, David E.; Kuhn, Michael; Prettner, Klaus (2024). "<a href="https://www.annualreviews.org/content/journals/10.1146/annurev-economics-081523-013750">Fertility in High-Income Countries: Trends, Patterns, Determinants, and Consequences</a>". <i>Annual Review of Economics</i>.</li></ul> <h2 id="further-reading">Further reading</h2> <p><i>This article incorporates material from the <a href="/facts/Citizendium/r8kIw6Hf">Citizendium</a> article "Fertility (demography)", which is licensed under the Creative Commons Attribution-ShareAlike 3.0 Unported License but not under the GFDL.</i> </p> <ul><li>Barrett RE, <a href="/facts/Donald_Bogue/xvKP9Sxs">Bogue DJ</a>, Anderton DL (1997). <i>The Population of the United States</i> (3rd ed.).</li> <li>Bock J (March 2002). <a href="https://doi.org/10.1002%2Fajhb.10039">"Introduction: evolutionary theory and the search for a unified theory of fertility"</a>. <i>American Journal of Human Biology</i>. 14 (2): 145–148. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1002%2Fajhb.10039">10.1002/ajhb.10039</a>. <a href="/facts/PMID_(identifier)/JlHAvMHt">PMID</a> <a href="https://pubmed.ncbi.nlm.nih.gov/11891930">11891930</a>. <a href="/facts/S2CID_(identifier)/ldJsHa2Y">S2CID</a> <a href="https://api.semanticscholar.org/CorpusID:27386441">27386441</a>.</li> <li>Campagne DM (January 2013). <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3850314">"Can Male Fertility Be Improved Prior to Assisted Reproduction through The Control of Uncommonly Considered Factors?"</a>. <i>International Journal of Fertility & Sterility</i>. 6 (4): 214–223. <a href="/facts/PMC_(identifier)/dX1zMt71">PMC</a> <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3850314">3850314</a>. <a href="/facts/PMID_(identifier)/JlHAvMHt">PMID</a> <a href="https://pubmed.ncbi.nlm.nih.gov/24520443">24520443</a>.</li> <li>Chavarro J (2009). <i>The Fertility Diet: Groundbreaking Research Reveals Natural Ways to Boost Ovulation and Improve Your Chances of Getting Pregnant</i>. McGraw-Hill Professional. <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 978-0-07-162710-8.</li> <li>Coale AJ, Watkin SC, eds. (1986). <i>The Decline of Fertility in Europe</i>.</li> <li><a href="/facts/Josef_Ehmer/uEo7V1kL">Ehrhardt J</a>, Kohli M, eds. (2011). <a href="https://web.archive.org/web/20130527141349/http://www.gesis.org/en/hsr/current-issues/current-issues-2010-2012/362-fertility/">"Special issue: Fertility in the History of the 20th Century: Trends, Theories, Policies, Discourses"</a>. <i><a href="/facts/Historical_Social_Research/wXDH3KYQ">Historical Social Research</a></i>. 36 (2). Archived from <a href="http://www.gesis.org/en/hsr/current-issues/current-issues-2010-2012/362-fertility/">the original</a> on 2013-05-27.</li> <li>Eversley DE (1959). <a href="https://web.archive.org/web/20090813022835/http://www.questia.com/read/27821849"><i>Social Theories of Fertility and the Malthusian Debate</i></a>. Archived from <a href="https://www.questia.com/read/27821849">the original</a> on 2009-08-13. Retrieved 2017-08-27.</li> <li>Garrett E, Reid A, Schürer K, Szreter S (July 2001). <a href="https://web.archive.org/web/20101215182716/http://www.questia.com/read/105014031"><i>Changing family size in England and Wales: Place, class and demography, 1891–1911</i></a>. Cambridge University Press. Archived from <a href="https://www.questia.com/read/105014031">the original</a> on 2010-12-15. Retrieved 2017-08-27.</li> <li>Grabill WH, Kiser CV, Whelpton PK (1958). <a href="https://web.archive.org/web/20090813022840/http://www.questia.com/read/31149540"><i>The Fertility of American Women</i></a>. Archived from <a href="https://www.questia.com/read/31149540">the original</a> on 2009-08-13. Retrieved 2017-08-27.</li> <li>Guzmán J, Singh S, Rodríguez G, Pantelides E (1996). <a href="https://web.archive.org/web/20090813023040/http://www.questia.com/read/91845268"><i>The fertility transition in Latin America</i></a>. International Union for the Scientific Study of Population. Archived from <a href="https://www.questia.com/read/91845268">the original</a> on 2009-08-13. Retrieved 2017-08-27.</li> <li>Haines MR, Steckel RH, eds. (2000). <i>A Population History of North America</i>. Cambridge University Press.</li> <li>Hawes JM, Nybakken EI, eds. (1991). <i>American Families: a Research Guide and Historical Handbook</i>. Greenwood Press.</li> <li>Klein HS (2004). <i>A Population History of the United States</i>. Cambridge University Press.</li> <li>Knox PL, Bartels EH, Bohland JR, Holcomb B, Johnston RJ (2001). <i>The United States: A Contemporary Human Geography</i>. Longman UK/Wiley USA.</li> <li>Kohler HP (2001). <a href="https://web.archive.org/web/20090813022827/https://www.questia.com/read/110004723"><i>Fertility and Social Interaction: An Economic Perspective</i></a>. Archived from <a href="https://www.questia.com/read/110004723">the original</a> on 13 August 2009.</li> <li>Jones C (March 2008). "Ethical and legal conundrums of postmodern procreation". <i>International Journal of Gynaecology and Obstetrics</i>. 100 (3): 208–210. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1016%2Fj.ijgo.2007.09.031">10.1016/j.ijgo.2007.09.031</a>. <a href="/facts/PMID_(identifier)/JlHAvMHt">PMID</a> <a href="https://pubmed.ncbi.nlm.nih.gov/18062970">18062970</a>. <a href="/facts/S2CID_(identifier)/ldJsHa2Y">S2CID</a> <a href="https://api.semanticscholar.org/CorpusID:13370582">13370582</a>.</li> <li>Leete R (1999). <a href="https://web.archive.org/web/20090813023035/https://www.questia.com/read/77198881"><i>Dynamics of Values in Fertility Change</i></a>. Archived from <a href="https://www.questia.com/read/77198881">the original</a> on 13 August 2009.</li> <li>Lovett LL (2007). <i>Conceiving the Future: Pronatalism, Reproduction, and the Family in the United States, 1890–1938</i>.</li> <li>Mintz S, Kellogg S (1988). <i>Domestic Revolutions: a Social History of American Family Life</i>.</li> <li>Pampel FC, Peters HE (1995). "The Easterlin effect". <i>Annual Review of Sociology</i>. 21: 163–194. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1146%2Fannurev.so.21.080195.001115">10.1146/annurev.so.21.080195.001115</a>. <a href="/facts/PMID_(identifier)/JlHAvMHt">PMID</a> <a href="https://pubmed.ncbi.nlm.nih.gov/12291060">12291060</a>.</li> <li><a href="https://web.archive.org/web/20090604214506/http://prb.org/Reports/2004/PopulationHandbook5thedition.aspx"><i>Population Handbook</i></a> (5th ed.). Population Reference Bureau. 2004. Archived from <a href="http://prb.org/Reports/2004/PopulationHandbook5thedition.aspx">the original</a> on 2009-06-04.</li> <li>Reed J (1978). <i>From Private Vice to Public Virtue: The Birth Control Movement and American Society Since 1830</i>.</li> <li>Tarver JD (1996). <a href="https://web.archive.org/web/20090813022830/https://www.questia.com/read/26237427"><i>The Demography of Africa</i></a>. Archived from <a href="https://www.questia.com/read/26237427">the original</a> on 13 August 2009.</li> <li>Weeks JR (2007). <i>Population: An Introduction to Concepts and Issues</i> (10th ed.).</li></ul> <h2 id="external-links">External links</h2> <ul><li><a href="https://web.archive.org/web/20090831054611/http://www.hfea.gov.uk/fertility.html">"Fertility treatment and clinics in the UK"</a>. <i>Human Fertilisation and Embryology Authority (HFEA)</i>. Archived from <a href="http://www.hfea.gov.uk/fertility.html">the original</a> on 2009-08-31.</li> <li><a href="https://hdl.handle.net/2027/mdp.39015007261855">Population and the American future: the report</a>. <i>United States. Commission on Population Growth and the American Future.</i> (Report). Washington: The Superintendent of Documents, U.S. Government Printing Office. 1972. <a href="/facts/Hdl_(identifier)/rdebSxmC">hdl</a>:<a href="https://hdl.handle.net/2027%2Fmdp.39015007261855">2027/mdp.39015007261855</a>.</li> <li><a href="https://web.archive.org/web/20100709055156/http://esa.un.org/unpd/wpp2008/all-wpp-indicators_components.htm">"United Nations World Population Prospects, the 2008 Revision"</a>. Archived from <a href="http://esa.un.org/unpd/wpp2008/all-wpp-indicators_components.htm">the original</a> on 2010-07-09.</li> <li>Calder, Vanessa Brown, and Chelsea Follett (August 10, 2023). <a href="https://www.cato.org/sites/cato.org/files/2023-08/PA-955.pdf">Freeing American Families: Reforms to Make Family Life Easier and More Affordable</a>, <i>Policy Analysis</i> no. 955, Cato Institute, Washington, DC.</li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Frank O (27 September 2017). "The demography of fertility and infertility". www.gfmer.ch. <a href="http://www.gfmer.ch/Books/Reproductive_health/The_demography_of_fertility_and_infertility.html" target="_blank">http://www.gfmer.ch/Books/Reproductive_health/The_demography_of_fertility_and_infertility.html</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>Last JM. "Fecundity and Fertility". Encyclopedia of Public Health. Archived from the original on 11 August 2009 – via enotes.com. <a href="https://web.archive.org/web/20090811030144/http://www.enotes.com/public-health-encyclopedia/fecundity-fertility" target="_blank">https://web.archive.org/web/20090811030144/http://www.enotes.com/public-health-encyclopedia/fecundity-fertility</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>Wurtenberger, Wyndy; Solmorales, Sequoia; Snopkowski, Kristin (2023). "Fertility". Encyclopedia of Sexual Psychology and Behavior. Springer, Cham. pp. 1–13. doi:10.1007/978-3-031-08956-5_979-1. ISBN 978-3-031-08956-5. <a href="978-3-031-08956-5" target="_blank">978-3-031-08956-5</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>"Dictionary of Medical Terms". familydoctor.org. Retrieved 2024-06-25. <a href="https://familydoctor.org/your-health-resources/health-tools/dictionary/" target="_blank">https://familydoctor.org/your-health-resources/health-tools/dictionary/</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>Gnoth, C.; Godehardt, E.; Frank-Herrmann, P.; Friol, K.; Tigges, Jürgen; Freundl, G. (2005-05-01). "Definition and prevalence of subfertility and infertility". Human Reproduction. 20 (5): 1144–1147. doi:10.1093/humrep/deh870. ISSN 1460-2350. PMID 15802321. <a href="http://academic.oup.com/humrep/article/20/5/1144/2356853/Definition-and-prevalence-of-subfertility-and" target="_blank">http://academic.oup.com/humrep/article/20/5/1144/2356853/Definition-and-prevalence-of-subfertility-and</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>"Infertility | Reproductive Health | CDC". www.cdc.gov. 2022-03-03. Retrieved 2023-03-02. <a href="https://www.cdc.gov/reproductivehealth/infertility/index.htm" target="_blank">https://www.cdc.gov/reproductivehealth/infertility/index.htm</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>Frank O (27 September 2017). "The demography of fertility and infertility". www.gfmer.ch. <a href="http://www.gfmer.ch/Books/Reproductive_health/The_demography_of_fertility_and_infertility.html" target="_blank">http://www.gfmer.ch/Books/Reproductive_health/The_demography_of_fertility_and_infertility.html</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> <li id="fn:8"><p>Last JM. "Fecundity and Fertility". Encyclopedia of Public Health. Archived from the original on 11 August 2009 – via enotes.com. <a href="https://web.archive.org/web/20090811030144/http://www.enotes.com/public-health-encyclopedia/fecundity-fertility" target="_blank">https://web.archive.org/web/20090811030144/http://www.enotes.com/public-health-encyclopedia/fecundity-fertility</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li> <li id="fn:9"><p>For detailed discussions of each measure see Paul George Demeny and Geoffrey McNicoll, Encyclopedia of Population (2003) <a href="#fnref:9" class="footnote-back-ref">↩</a></p></li> <li id="fn:10"><p>Another way of doing it is to add up the ASFR for age 10-14, 15-19, 20-24, etc., and multiply by 5 (to cover the 5 year interval). <a href="#fnref:10" class="footnote-back-ref">↩</a></p></li> <li id="fn:11"><p>Murphy M (3 July 2013). "Cross-national patterns of intergenerational continuities in childbearing in developed countries". Biodemography and Social Biology. 59 (2): 101–126. doi:10.1080/19485565.2013.833779. PMC 4160295. PMID 24215254. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4160295" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4160295</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></p></li> <li id="fn:12"><p>Hayford SR, Morgan SP (1 March 2008). "Religiosity and Fertility in the United States: The Role of Fertility Intentions". Social Forces. 86 (3): 1163–1188. doi:10.1353/sof.0.0000. PMC 2723861. PMID 19672317. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2723861" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2723861</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></p></li> <li id="fn:13"><p>Dommermuth L, Klobas J, Lappegård T (2014). Differences in childbearing by time frame of fertility intention. A study using survey and register data from Norway (Report). hdl:10419/192763. <a href="/wiki/Hdl_(identifier)" target="_blank">/wiki/Hdl_(identifier)</a> <a href="#fnref:13" class="footnote-back-ref">↩</a></p></li> <li id="fn:14"><p>Schaffnit SB, Sear R (1 July 2014). "Wealth modifies relationships between kin and women's fertility in high-income countries". Behavioral Ecology. 25 (4): 834–842. doi:10.1093/beheco/aru059. <a href="https://doi.org/10.1093%2Fbeheco%2Faru059" target="_blank">https://doi.org/10.1093%2Fbeheco%2Faru059</a> <a href="#fnref:14" class="footnote-back-ref">↩</a></p></li> <li id="fn:15"><p>Rai PK, Pareek S, Joshi H (1 January 2013). "Regression Analysis of Collinear Data using r-k Class Estimator: Socio-Economic and Demographic Factors Affecting the Total Fertility Rate (TFR) in India". Journal of Data Science. 11 (2): 323–342. doi:10.6339/JDS.2013.11(2).1030. <a href="https://doi.org/10.6339%2FJDS.2013.11%282%29.1030" target="_blank">https://doi.org/10.6339%2FJDS.2013.11%282%29.1030</a> <a href="#fnref:15" class="footnote-back-ref">↩</a></p></li> <li id="fn:16"><p>"Health and Education". Project Drawdown. 2020-02-12. Archived from the original on 2020-11-29. Retrieved 2020-12-04. <a href="https://web.archive.org/web/20201129190030/https://drawdown.org/solutions/health-and-education/technical-summary" target="_blank">https://web.archive.org/web/20201129190030/https://drawdown.org/solutions/health-and-education/technical-summary</a> <a href="#fnref:16" class="footnote-back-ref">↩</a></p></li> <li id="fn:17"><p>Bloom D, Canning D, Fink G, Finlay J (2009). "Fertility, female labor force participation, and the demographic dividend". Journal of Economic Growth. 14 (2): 79–101. doi:10.1007/s10887-009-9039-9. <a href="https://doi.org/10.1007%2Fs10887-009-9039-9" target="_blank">https://doi.org/10.1007%2Fs10887-009-9039-9</a> <a href="#fnref:17" class="footnote-back-ref">↩</a></p></li> <li id="fn:18"><p>Sato Y (March 2007). "Economic geography, fertility and migration". Journal of Urban Economics. 61 (2): 372–387. doi:10.1016/j.jue.2006.08.002. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:18" class="footnote-back-ref">↩</a></p></li> <li id="fn:19"><p>Li A (November 2019). "Fertility intention-induced relocation: The mediating role of housing markets". Population, Space and Place. 25 (8). doi:10.1002/psp.2265. hdl:11343/286897. S2CID 200012816. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:19" class="footnote-back-ref">↩</a></p></li> <li id="fn:20"><p>Atalay K, Li A, Whelan S (25 June 2021). "Housing wealth, fertility intentions and fertility". Journal of Housing Economics. 54: 101787. doi:10.1016/j.jhe.2021.101787. S2CID 237787783. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:20" class="footnote-back-ref">↩</a></p></li> <li id="fn:21"><p>Bongaarts J (March 1978). "A Framework for Analyzing the Proximate Determinants of Fertility". Population and Development Review. 4 (1): 105–132. doi:10.2307/1972149. JSTOR 1972149. S2CID 27026630. <a href="/wiki/John_Bongaarts" target="_blank">/wiki/John_Bongaarts</a> <a href="#fnref:21" class="footnote-back-ref">↩</a></p></li> <li id="fn:22"><p>Stover J (September 1998). "Revising the proximate determinants of fertility framework: what have we learned in the past 20 years?". Studies in Family Planning. 29 (3): 255–267. doi:10.2307/172272. JSTOR 172272. PMID 9789319. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:22" class="footnote-back-ref">↩</a></p></li> <li id="fn:23"><p>Becker GS (1960). An Economic Analysis of Fertility. National Bureau Committee for Economic Research, Demographic and Economic Change in Developed Countries, a Conference of the Universities. Princeton, N.J.: Princeton University Press. <a href="#fnref:23" class="footnote-back-ref">↩</a></p></li> <li id="fn:24"><p>Mincer J (1963). "Market Prices, Opportunity Costs, and Income Effects". In Christ C (ed.). Measurement in Economics. Stanford, CA: Stanford University Press. <a href="#fnref:24" class="footnote-back-ref">↩</a></p></li> <li id="fn:25"><p>Easterlin RA (March 1975). "An economic framework for fertility analysis". Studies in Family Planning. 6 (3): 54–63. doi:10.2307/1964934. JSTOR 1964934. PMID 1118873. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:25" class="footnote-back-ref">↩</a></p></li> <li id="fn:26"><p>Zegers-Hochschild, Fernando; Adamson, G David; Dyer, Silke; Racowsky, Catherine; de Mouzon, Jacques; Sokol, Rebecca; Rienzi, Laura; Sunde, Arne; Schmidt, Lone; Cooke, Ian D; Simpson, Joe Leigh; van der Poel, Sheryl (Sep 2017). "The International Glossary on Infertility and Fertility Care, 2017". Human Reproduction (Oxford, England). 32 (9): 1786–1801. doi:10.1093/humrep/dex234. ISSN 0268-1161. PMC 5850297. PMID 29117321. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5850297" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5850297</a> <a href="#fnref:26" class="footnote-back-ref">↩</a></p></li> <li id="fn:27"><p>Skakkebæk NE, Lindahl-Jacobsen R, Levine H, Andersson AM, Jørgensen N, Main KM, et al. (March 2022). "Environmental factors in declining human fertility". Nature Reviews. Endocrinology. 18 (3): 139–157. doi:10.1038/s41574-021-00598-8. PMID 34912078. S2CID 245134205. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:27" class="footnote-back-ref">↩</a></p></li> <li id="fn:28"><p>George K, Kamath MS (September 2010). "Fertility and age". Journal of Human Reproductive Sciences. 3 (3): 121–123. doi:10.4103/0974-1208.74152. PMC 3017326. PMID 21234171. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3017326" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3017326</a> <a href="#fnref:28" class="footnote-back-ref">↩</a></p></li> <li id="fn:29"><p>"How to get pregnant". Mayo Clinic. 2016-11-02. Retrieved 2018-02-16. <a href="https://www.mayoclinic.org/healthy-lifestyle/getting-pregnant/in-depth/how-to-get-pregnant/art-20047611" target="_blank">https://www.mayoclinic.org/healthy-lifestyle/getting-pregnant/in-depth/how-to-get-pregnant/art-20047611</a> <a href="#fnref:29" class="footnote-back-ref">↩</a></p></li> <li id="fn:30"><p>"Fertility problems: assessment and treatment, Clinical guideline [CG156]". National Institute for Health and Care Excellence. 20 February 2013. Retrieved 2018-02-16. publication date: February 2013. last updated: September 2017 <a href="https://www.nice.org.uk/guidance/cg156/ifp/chapter/trying-for-a-baby" target="_blank">https://www.nice.org.uk/guidance/cg156/ifp/chapter/trying-for-a-baby</a> <a href="#fnref:30" class="footnote-back-ref">↩</a></p></li> <li id="fn:31"><p>Imler PB, Wilbanks D. "The Essential Guide to Getting Pregnant" (PDF). American Pregnancy Association. Archived from the original (PDF) on 2018-06-01. Retrieved 2018-02-16. <a href="https://web.archive.org/web/20180601033115/http://americanpregnancy.org/getting-pregnant-ebook/p7M7O0q1c71703C/gettingpregnant.pdf" target="_blank">https://web.archive.org/web/20180601033115/http://americanpregnancy.org/getting-pregnant-ebook/p7M7O0q1c71703C/gettingpregnant.pdf</a> <a href="#fnref:31" class="footnote-back-ref">↩</a></p></li> <li id="fn:32"><p>Danilovich N, Babu PS, Xing W, Gerdes M, Krishnamurthy H, Sairam MR (November 2000). "Estrogen deficiency, obesity, and skeletal abnormalities in follicle-stimulating hormone receptor knockout (FORKO) female mice". Endocrinology. 141 (11): 4295–4308. doi:10.1210/endo.141.11.7765. PMID 11089565. <a href="https://doi.org/10.1210%2Fendo.141.11.7765" target="_blank">https://doi.org/10.1210%2Fendo.141.11.7765</a> <a href="#fnref:32" class="footnote-back-ref">↩</a></p></li> <li id="fn:33"><p>"My Fertile Period | DuoFertility". Archived from the original on 2008-12-21. Retrieved 2008-09-22. <a href="https://web.archive.org/web/20081221225600/http://www.duofertility.com/en/my-body/my-cycle/my-fertile-period.html" target="_blank">https://web.archive.org/web/20081221225600/http://www.duofertility.com/en/my-body/my-cycle/my-fertile-period.html</a> <a href="#fnref:33" class="footnote-back-ref">↩</a></p></li> <li id="fn:34"><p>Creinin MD, Keverline S, Meyn LA (October 2004). "How regular is regular? An analysis of menstrual cycle regularity". Contraception. 70 (4): 289–292. doi:10.1016/j.contraception.2004.04.012. PMID 15451332. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:34" class="footnote-back-ref">↩</a></p></li> <li id="fn:35"><p>Anderson SE, Dallal GE, Must A (April 2003). "Relative weight and race influence average age at menarche: results from two nationally representative surveys of US girls studied 25 years apart". Pediatrics. 111 (4 Pt 1): 844–850. doi:10.1542/peds.111.4.844. PMID 12671122. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:35" class="footnote-back-ref">↩</a></p></li> <li id="fn:36"><p>Apter D (February 1980). "Serum steroids and pituitary hormones in female puberty: a partly longitudinal study". Clinical Endocrinology. 12 (2): 107–120. doi:10.1111/j.1365-2265.1980.tb02125.x. PMID 6249519. S2CID 19913395. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:36" class="footnote-back-ref">↩</a></p></li> <li id="fn:37"><p>Takahashi TA, Johnson KM (May 2015). "Menopause". The Medical Clinics of North America. 99 (3): 521–534. doi:10.1016/j.mcna.2015.01.006. PMID 25841598. <a href="https://doi.org/10.1016%2Fj.mcna.2015.01.006" target="_blank">https://doi.org/10.1016%2Fj.mcna.2015.01.006</a> <a href="#fnref:37" class="footnote-back-ref">↩</a></p></li> <li id="fn:38"><p>Bourgeois JF, Gehrig PA, Veljovich DS (1 January 2005). Obstetrics and Gynecology Recall. Lippincott Williams & Wilkins. ISBN 9780781748797 – via Google Books. <a href="9780781748797" target="_blank">9780781748797</a> <a href="#fnref:38" class="footnote-back-ref">↩</a></p></li> <li id="fn:39"><p>A computer simulation run by Henri Leridon, PhD, an epidemiologist with the French Institute of Health and Medical Research: Leridon H (July 2004). "Can assisted reproduction technology compensate for the natural decline in fertility with age? A model assessment". Human Reproduction. 19 (7): 1548–1553. doi:10.1093/humrep/deh304. PMID 15205397. <a href="https://doi.org/10.1093%2Fhumrep%2Fdeh304" target="_blank">https://doi.org/10.1093%2Fhumrep%2Fdeh304</a> <a href="#fnref:39" class="footnote-back-ref">↩</a></p></li> <li id="fn:40"><p>A computer simulation run by Henri Leridon, PhD, an epidemiologist with the French Institute of Health and Medical Research: Leridon H (July 2004). "Can assisted reproduction technology compensate for the natural decline in fertility with age? A model assessment". Human Reproduction. 19 (7): 1548–1553. doi:10.1093/humrep/deh304. PMID 15205397. <a href="https://doi.org/10.1093%2Fhumrep%2Fdeh304" target="_blank">https://doi.org/10.1093%2Fhumrep%2Fdeh304</a> <a href="#fnref:40" class="footnote-back-ref">↩</a></p></li> <li id="fn:41"><p>Dunson DB, Baird DD, Colombo B (January 2004). "Increased infertility with age in men and women". Obstetrics and Gynecology. 103 (1): 51–56. doi:10.1097/01.AOG.0000100153.24061.45. PMID 14704244. S2CID 23061073. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:41" class="footnote-back-ref">↩</a></p></li> <li id="fn:42"><p>Rothman KJ, Wise LA, Sørensen HT, Riis AH, Mikkelsen EM, Hatch EE (June 2013). "Volitional determinants and age-related decline in fecundability: a general population prospective cohort study in Denmark". Fertility and Sterility. 99 (7): 1958–1964. doi:10.1016/j.fertnstert.2013.02.040. PMC 3672329. PMID 23517858. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3672329" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3672329</a> <a href="#fnref:42" class="footnote-back-ref">↩</a></p></li> <li id="fn:43"><p>ASRM Practice Committee (September 2013). "Optimizing natural fertility: a committee opinion". Fertility and Sterility. 100 (3): 631–637. doi:10.1016/j.fertnstert.2013.07.011. PMID 23993665. <a href="https://doi.org/10.1016%2Fj.fertnstert.2013.07.011" target="_blank">https://doi.org/10.1016%2Fj.fertnstert.2013.07.011</a> <a href="#fnref:43" class="footnote-back-ref">↩</a></p></li> <li id="fn:44"><p>"Spanish woman 'is oldest mother'". BBC News. 2006-12-30. Retrieved 2006-12-30. <a href="http://news.bbc.co.uk/2/hi/health/6220523.stm" target="_blank">http://news.bbc.co.uk/2/hi/health/6220523.stm</a> <a href="#fnref:44" class="footnote-back-ref">↩</a></p></li> <li id="fn:45"><p>Kidd SA, Eskenazi B, Wyrobek AJ (February 2001). "Effects of male age on semen quality and fertility: a review of the literature". Fertility and Sterility. 75 (2): 237–248. doi:10.1016/S0015-0282(00)01679-4. PMID 11172821. <a href="https://doi.org/10.1016%2FS0015-0282%2800%2901679-4" target="_blank">https://doi.org/10.1016%2FS0015-0282%2800%2901679-4</a> <a href="#fnref:45" class="footnote-back-ref">↩</a></p></li> <li id="fn:46"><p>Kidd SA, Eskenazi B, Wyrobek AJ (February 2001). "Effects of male age on semen quality and fertility: a review of the literature". Fertility and Sterility. 75 (2): 237–248. doi:10.1016/S0015-0282(00)01679-4. PMID 11172821. <a href="https://doi.org/10.1016%2FS0015-0282%2800%2901679-4" target="_blank">https://doi.org/10.1016%2FS0015-0282%2800%2901679-4</a> <a href="#fnref:46" class="footnote-back-ref">↩</a></p></li> <li id="fn:47"><p>Kidd SA, Eskenazi B, Wyrobek AJ (February 2001). "Effects of male age on semen quality and fertility: a review of the literature". Fertility and Sterility. 75 (2): 237–248. doi:10.1016/S0015-0282(00)01679-4. PMID 11172821. <a href="https://doi.org/10.1016%2FS0015-0282%2800%2901679-4" target="_blank">https://doi.org/10.1016%2FS0015-0282%2800%2901679-4</a> <a href="#fnref:47" class="footnote-back-ref">↩</a></p></li> <li id="fn:48"><p>Silber S (August 1991). "Effect of Age on Male Fertility". Seminars in Reproductive Medicine. 9 (3): 241–248. doi:10.1055/s-2007-1019415. S2CID 84196394. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:48" class="footnote-back-ref">↩</a></p></li> <li id="fn:49"><p>Campagne DM (January 2013). "Can Male Fertility Be Improved Prior to Assisted Reproduction through The Control of Uncommonly Considered Factors?". International Journal of Fertility & Sterility. 6 (4): 214–223. PMC 3850314. PMID 24520443. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3850314" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3850314</a> <a href="#fnref:49" class="footnote-back-ref">↩</a></p></li> <li id="fn:50"><p>Wiener-Megnazi Z, Auslender R, Dirnfeld M (January 2012). "Advanced paternal age and reproductive outcome". Asian Journal of Andrology. 14 (1): 69–76. doi:10.1038/aja.2011.69. PMC 3735149. PMID 22157982. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3735149" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3735149</a> <a href="#fnref:50" class="footnote-back-ref">↩</a></p></li> <li id="fn:51"><p>Reichenberg A, Gross R, Weiser M, Bresnahan M, Silverman J, Harlap S, et al. (September 2006). "Advancing paternal age and autism". Archives of General Psychiatry. 63 (9): 1026–1032. doi:10.1001/archpsyc.63.9.1026. PMID 16953005. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:51" class="footnote-back-ref">↩</a></p></li> <li id="fn:52"><p>Jaffe AE, Eaton WW, Straub RE, Marenco S, Weinberger DR (March 2014). "Paternal age, de novo mutations and schizophrenia". Molecular Psychiatry. 19 (3): 274–275. doi:10.1038/mp.2013.76. PMC 3929531. PMID 23752248. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3929531" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3929531</a> <a href="#fnref:52" class="footnote-back-ref">↩</a></p></li> <li id="fn:53"><p>Schulz SC, Green MF, Nelson KJ (1 April 2016). Schizophrenia and Psychotic Spectrum Disorders. Oxford University Press. ISBN 9780199378074 – via Google Books. <a href="9780199378074" target="_blank">9780199378074</a> <a href="#fnref:53" class="footnote-back-ref">↩</a></p></li> <li id="fn:54"><p>Malaspina D, Harlap S, Fennig S, Heiman D, Nahon D, Feldman D, Susser ES (April 2001). "Advancing paternal age and the risk of schizophrenia". Archives of General Psychiatry. 58 (4): 361–367. doi:10.1001/archpsyc.58.4.361. PMID 11296097. <a href="https://doi.org/10.1001%2Farchpsyc.58.4.361" target="_blank">https://doi.org/10.1001%2Farchpsyc.58.4.361</a> <a href="#fnref:54" class="footnote-back-ref">↩</a></p></li> <li id="fn:55"><p>Sipos A, Rasmussen F, Harrison G, Tynelius P, Lewis G, Leon DA, Gunnell D (November 2004). "Paternal age and schizophrenia: a population based cohort study". BMJ. 329 (7474): 1070. doi:10.1136/bmj.38243.672396.55. PMC 526116. PMID 15501901. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC526116" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC526116</a> <a href="#fnref:55" class="footnote-back-ref">↩</a></p></li> <li id="fn:56"><p>Malaspina D, Corcoran C, Fahim C, Berman A, Harkavy-Friedman J, Yale S, et al. (April 2002). "Paternal age and sporadic schizophrenia: evidence for de novo mutations". American Journal of Medical Genetics. 114 (3): 299–303. doi:10.1002/ajmg.1701. PMC 2982144. PMID 11920852. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2982144" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2982144</a> <a href="#fnref:56" class="footnote-back-ref">↩</a></p></li> <li id="fn:57"><p>"Obesity | Fat men linked to low fertility". Sydney Morning Herald. 18 October 2010. Retrieved 19 October 2010. <a href="http://www.smh.com.au/lifestyle/wellbeing/fat-men-linked-to-lower-fertility-20101017-16p6l.html" target="_blank">http://www.smh.com.au/lifestyle/wellbeing/fat-men-linked-to-lower-fertility-20101017-16p6l.html</a> <a href="#fnref:57" class="footnote-back-ref">↩</a></p></li> <li id="fn:58"><p>Plas E, Berger P, Hermann M, Pflüger H (August 2000). "Effects of aging on male fertility?". Experimental Gerontology. 35 (5): 543–551. doi:10.1016/S0531-5565(00)00120-0. PMID 10978677. S2CID 19875208. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:58" class="footnote-back-ref">↩</a></p></li> <li id="fn:59"><p>Age Limit of Sperm Donors in the United Kingdom Pdf file Archived October 3, 2008, at the Wayback Machine <a href="http://www.spermdonorforum.com/documents/sperm_donor_clinics_uk_2007.pdf" target="_blank">http://www.spermdonorforum.com/documents/sperm_donor_clinics_uk_2007.pdf</a> <a href="#fnref:59" class="footnote-back-ref">↩</a></p></li> <li id="fn:60"><p>Arteaga O, Maoz BM, Nichols S, Markovich G, Kahr B (June 2014). "Complete polarimetry on the asymmetric transmission through subwavelength hole arrays". Optics Express. 22 (11): 13719–13732. doi:10.2307/286642. JSTOR 286642. PMID 24921565. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:60" class="footnote-back-ref">↩</a></p></li> <li id="fn:61"><p>Pineau E (2020-01-23). "French Senate approves bill allowing IVF for single women, lesbians". Reuters. Retrieved 2021-04-21. <a href="https://www.reuters.com/article/us-france-ivf-idUSKBN1ZL33Y" target="_blank">https://www.reuters.com/article/us-france-ivf-idUSKBN1ZL33Y</a> <a href="#fnref:61" class="footnote-back-ref">↩</a></p></li> <li id="fn:62"><p>Ahn, Ashley (19 March 2023). "South Korea has the world's lowest fertility rate, a struggle with lessons for us all". NPR. <a href="https://www.npr.org/2023/03/19/1163341684/south-korea-fertility-rate" target="_blank">https://www.npr.org/2023/03/19/1163341684/south-korea-fertility-rate</a> <a href="#fnref:62" class="footnote-back-ref">↩</a></p></li> <li id="fn:63"><p>Anderson, Thomas; Kohler, Hans-Peter (2013). "Education Fever and the East Asian Fertility Puzzle". Asian Population Studies. 9 (2): 196–215. doi:10.1080/17441730.2013.797293. PMC 4038909. PMID 24883076. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4038909" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4038909</a> <a href="#fnref:63" class="footnote-back-ref">↩</a></p></li> <li id="fn:64"><p>https://core.ac.uk/download/pdf/51182325.pdf [bare URL PDF] <a href="https://core.ac.uk/download/pdf/51182325.pdf" target="_blank">https://core.ac.uk/download/pdf/51182325.pdf</a> <a href="#fnref:64" class="footnote-back-ref">↩</a></p></li> <li id="fn:65"><p>Kim, Sunhye (2019). "Reproductive technologies as population control: How pronatalist policies harm reproductive health in South Korea". Sexual and Reproductive Health Matters. 27 (2): 6–12. doi:10.1080/26410397.2019.1610278. PMC 7888060. PMID 31533588. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7888060" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7888060</a> <a href="#fnref:65" class="footnote-back-ref">↩</a></p></li> <li id="fn:66"><p>Oppenheimer VK. Women's rising employment and the future of the family in industrial societies. OCLC 936722012. <a href="/wiki/OCLC_(identifier)" target="_blank">/wiki/OCLC_(identifier)</a> <a href="#fnref:66" class="footnote-back-ref">↩</a></p></li> <li id="fn:67"><p>Bailey MJ (1 February 2006). "More Power to the Pill: The Impact of Contraceptive Freedom on Women's Life Cycle Labor Supply". The Quarterly Journal of Economics. 121 (1): 289–320. doi:10.1093/qje/121.1.289. hdl:1803/15760. PMC 11929136. PMID 40124738. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC11929136" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC11929136</a> <a href="#fnref:67" class="footnote-back-ref">↩</a></p></li> </ol>