Age and female fertility

Quantification of effect

In adolescence

The average age of a girl's first period (menarche) is 12 to 13 with regional variations: 12.5 years in the United States, 12.72 in Canada, 12.9 in the UK).

In the first year post-menarche, around 80% of cycles are anovulatory. This declines to 50% in the third year, and to 10% by the sixth. Little is known about fertility in young adolescents, as early teenage pregnancies are uncommon in most societies.

In adulthood

Female fertility generally peaks between the late teens and late twenties, after which it starts to decline. However, the probability of conception at a specific age varies between sources and is subject to debate.

According to the National Institute for Health and Care Excellence (NICE), over 80% of women aged under 40 who have regular unprotected sexual intercourse will become pregnant within one year of trying. In the second year this percentage rises to over 90%.

A 2004 study by 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, had the following results on rates of conception by age:

• 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 aged 19–39, fertility starts declining after age 27 and drops at a somewhat greater rate after age 35. Statistical analysis showed that the women in the 27–29 age group had a significantly lower chance on average of becoming pregnant than did the 19–26-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 many variables that were too difficult to sort out, for a clear conclusion to be drawn. Some experts suggested that the main change in fertility in older women was that it took them longer to conceive, not necessarily that they were significantly less likely 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 aged 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.

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 following is a table containing estimates of the percentage of women who will fail to obtain a live birth by the age of the woman when she starts trying to conceive. Note that while for the young ages researchers tend to agree, for older ages there is discrepancy.

Ovarian reserve

In terms of ovarian reserve, a typical woman has 12% of her reserve at age 30 and has only 3% at age 40. 81% of variation in ovarian reserve is due to age alone, making age the most important factor in female infertility.

The most common methods of checking the status of the ovarian reserve are to perform a blood test on day 3 of the menstrual cycle to measure serum follicle-stimulating hormone (FSH) level. Alternatively, a blood test to measure the serum anti-müllerian hormone (AMH) level can give similar information. Transvaginal ultrasound can also be used to "count the number of follicles" and this procedure is called antral Follicle Count.

The American College of Obstetricians and Gynecologists recommends ovarian reserve testing should be performed for women older than 35 years who have not conceived after six months of attempting pregnancy and women at higher risk of diminished ovarian reserve, such as those with a history of cancer treated with gonadotoxic therapy, pelvic irradiation, or both; those with medical conditions that were treated with gonadotoxic therapies; or those who had ovarian surgery for endometriomas.

A poor result from ovarian reserve testing does not signify an absolute inability to conceive and should not be the sole criterion considered to limit or deny access to infertility treatment.

Historical data

A study of a population of French women from 1670 and 1789 shows that those who married at age 20–24 had 7.0 children on average and 3.7% remained childless. Women who married at age 25–29 years had a mean of 5.7 children and 5.0% remained childless. Women who married at 30–34 years had a mean of 4.0 children and 8.2% remained childless. The average age at last birth in natural fertility populations that have been studied is around 40.

In 1957, a study was done on a large population (American Hutterites) that never used birth control. The investigators measured the relationship between the age of the female partner and fertility. (Infertility rates today are believed to be higher in the general population than for the population in this study from the 1950s.)

This 1957 study found that:

• By age 30, 7% of couples were infertile
• By age 35, 11% of couples were infertile
• By age 40, 33% of couples were infertile
• At age 45, 87% of couples were infertile