Female Age Factors

A woman’s fertility begins to decline years before menopause, even if ovulation and menstrual cycles remain normal. Numerous demographic and epidemiological studies have long observed this phenomenon of declining fertility with age. A classic study by the North American Hutterites precisely revealed the decline in female fertility with age, particularly emphasizing a significant drop after age 40. This theory, introduced to the United States from Switzerland in the 1870s, is now accepted in parts of Dakota, Montana, and Canada. The Hutterites’ research promoted larger family sizes and condemned any form of contraception, resulting in one of the highest historical birth rates, with an average of 11 deliveries per married woman. Among the 209 women studied, only five were childless, giving an infertility rate of 2.4%. The decline in fertility is notable in older women: 11% of women over 34 years old did not conceive again, 33% of women over 40 did not conceive again, and the infertility rate for women over 45 was as high as 87%. The average age for the last pregnancy was 40.9 years.

The Hutterites’ data, along with other epidemiological studies, indicate that fertility declines with age. The fertility peak is around 20 years old, begins to decline at age 30, accelerates after age 35, drops significantly after age 40, and pregnancies are rare after age 45.

An analysis of historical census data showed that the proportion of women who failed to conceive without contraception increased steadily with age: 6% at ages 20-24, 9% at ages 25-29, 15% at ages 30-34, 30% at ages 35-39, and 64% at ages 40-44.

Female reproductive cells are not replenished throughout life. The number of oocytes and follicles is determined during fetal development, and their quantity declines exponentially from mid-gestation to menopause. As women age, the quality of remaining oocytes also decreases, and the frequency of sexual intercourse declines year by year.

Male Age Factors

Although the frequency of sexual intercourse decreases significantly with age, this is not the sole reason for declining female fertility. Another less frequently mentioned factor is male age. As men age, there is a decline in semen volume and sperm motility, and an increase in abnormal sperm morphology, though sperm concentration does not decrease. Based on these changes, it can be inferred that male fertility also declines with age, especially after age 50. However, data show that fertility declines significantly in women before age 50. There is no evidence to suggest a specific age at which men can no longer conceive, so fertility primarily depends on the woman’s age rather than the man’s.

A French control trial studied male factors (including age) and intercourse frequency in women undergoing artificial insemination due to male infertility. After 12 cycles of artificial insemination, the cumulative pregnancy rate for women under 30 was 73%, 62% for women aged 31-35, and 54% for women aged 36-40. The decline in fertility in women is related to reduced intercourse frequency but is mainly influenced by the woman’s age.

ART Success Rate and Female Age

The success rate of Assisted Reproductive Technology (ART) also decreases with increasing female age. Female age is the primary factor influencing the chance of a live birth using one’s own oocytes. For women in their twenties, pregnancy rates, live birth rates, and singleton live birth rates are relatively stable. However, from age 35, these rates begin to decline steadily. According to the 2005 ART success rates, the live birth rate per treatment cycle was 37% for women under 35, 30% for women aged 35-37, 20% for women aged 38-40, 11% for women aged 41-42, and about 4% for women over 43.

As women’s age increases, the proportion of abnormal embryos also increases, affecting fertility. Data show that among embryos appearing normal after IVF, 39% of embryos from women aged 40 or older were abnormal according to fluorescence in situ hybridization, compared to only 5% of embryos from women aged 20-24. The live birth rate for IVF with donor eggs indicates that the recipient’s age does not significantly affect the live birth rate.

The incidence of miscarriage also increases with the patient’s age. The decline in female fertility and early pregnancy loss is largely due to abnormal oocytes. Studies indicate that in older women, the normal spindle formation process involved in cell division is significantly altered, leading to a higher incidence of aneuploidy. The rate of aneuploidy in oocytes and embryos increases significantly with age, contributing to the higher spontaneous miscarriage rate and lower live birth rate in older ART patients. Reliable data estimate that the aneuploidy rate in oocytes is relatively low (<10%) for women under 35, about 30% at age 40, 50% at age 42, and nearly 100% at age 45.