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Female fertility is affected by age. After puberty, female fertility increases and then decreases, with advanced maternal age causing an increased risk of female infertility. In humans, a woman's fertility peaks in the early and mid-20s, after which it starts to decline slowly, with a more dramatic drop at around 35.〔 Menopause, or the cessation of menstrual periods, generally occurs in the 40s and 50s and marks the cessation of fertility, although age-related infertility can occur before then. The relationship between age and female fertility is popularly referred to as a woman's "biological clock"; when a woman reaches an age where fertility is commonly understood to drop, it can be said that her "biological clock is ticking." ==Quantification of effect== The average age of a young woman's first period (menarche) is 12 to 13 (12.5 years in the United States, 12.72 in Canada, According to Henri Leridon, PhD, an epidemiologist with the French Institute of Health and Medical Research of women trying to get pregnant, without using fertility drugs or in vitro fertilization〔()() 〕 *At age 30 * *75% will have a conception ending in a live birth within one year * *91% will have a conception ending in a live birth within four years *At age 35 * * 66% will have a conception ending in a live birth within one year * * 84% will have a conception ending in a live birth within four years *At age 40 * *44% will have a conception ending in a live birth within one year * *64% will have a conception ending in a live birth within four years〔 According to a study done on a sample of 782 healthy European couples ages 19–39, fertility starts declining after age 27 and drops at a somewhat greater rate after age 35. The women were divided into four age groups: 19–26, 27–29, 30–34 and 35–39. Statistical analysis showed that the women in the 27–29 age group had significantly less chance on average of becoming pregnant than did the 19- to 26-year-olds. Pregnancy rates did not change notably between the 27–29 age group and the 30–34 age group, but dropped significantly for the 35-39 age group. The age of the male partner had a significant impact on female fertility among the women who had reached their mid-30s, but not among the younger women. However, experts said the new study was too small and there were too many variables which were too difficult to sort out, for a clear conclusion to be drawn. Some experts suggested that the main change in fertility in the older women was the fact that it took them ''longer'' to conceive, not necessary that they were significantly more unlikely to eventually succeed. David Dunson, a biostatistician at the U.S. National Institute of Environmental Health Sciences, said that: "Although we noted a decline in female fertility in the late 20s, what we found was a decrease in the probability of becoming pregnant per menstrual cycle, not in the probability of eventually achieving a pregnancy."〔 A French study found no difference between the fertility rate of women under 25 and those ages 26–30, after which fertility started to decrease. Estimating the "fertility of a woman" is quite difficult because of the male factor (quality of sperm). This French study looked at 2,193 women who were using artificial insemination because their husbands were azoospermic. The cumulative success rates after 12 cycles of insemination were 73% for women under age 25, 74% in women ages 26–30, 61% for ages 31–35, and 54% in the over 35 age group. (Note that the study is from 1982; artificial insemination techniques and success rates have evolved greatly since then.) In Hungary, a study by the Központi Statisztikai Hivatal (Central Statistics Office) estimated that 7%–12% of Hungarian women younger than 30 were infertile; 13%–22% of women age 35 were infertile; and 24%–46% of women age 40 were infertile. The below is a table containing estimates of the percentage of women who, if starting to conceive at a certain age, will fail to obtain a live birth. Note that while for the young ages researchers tend to agree, for older ages there is discrepancy. As observed in the table above, there isn't much agreement about the ability of women in their forties to have a live birth. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Age and female fertility」の詳細全文を読む スポンサード リンク
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