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Onset of capacity for childbearing in women is dated biologically by menarche, although actual onset may be delayed. The end of childbearing is less understood but recent demographic and biological research on fertility at older ages in clarifying the end of fertility. The demographic view of declining fertility with age is based on age-specific fertility in natural fertility populations, artificial insemination and pregnancy rates by age and World Fertility Survey data.
New data from the Demographic and Health Surveys on exposure to the risk of pregnancy shows that whereas older women biologically need longer exposure to pregnancy, exposure declines on behavioural grounds such as duration of marriage. Actual fecundity is obscured by factors of fecundability.
Recent research on medically assisted conception is adding to the understanding of declining fecundity with age, especially the relative contributions of endometrial and ovarian ageing. This paper reviews the available information on declining fertility with age and discusses the implications of the extension of fertility through new medical technologies.
PIP: This literature review on fecundity and age interactions discusses studies on menopause, on artificial insemination, on sexual behavior, and on assisted reproductive technology. Challenges to the age old notion of biological reproductive declines with age have appeared since the advent of new technologies and the understanding of reproductive potential.
With donated oocytes, acyclic women can have a higher probability of conception than young women conceiving naturally. The beginning of childbearing is easily demarcated with the advent of menstrual cycles and fecundity with first ovulation. There are societal constraints on fecundity. Fixing a date on the end of childbearing is not as clear cut.
An understanding of the limits of childbearing comes from research on cessation of ovarian follicular function, on natural childbearing populations and ages of women at their last birth, and on achieved pregnancy among women artificially inseminated.
The end of menopause tends to occur during the mid-50s, but natural population data indicate that fecundity usually ends at around 39-41 years. The gap between fecundity and menopause is determined to be about 8-10 years. Findings from the artificial insemination literature indicate that conception rates decline for women older than 30 years.
Van Noord-Zaadastra finds that conceptions among women aged 31 years and older are 66% of those for women aged 21-30 years, after 12 cycles of insemination. Young women conceive in about 90% of cases in 24 cycles.
Comparisons with natural fertility indicate that use of fresh sperm increases pregnancy rates in cases of insemination. Sex behavior surveys find that coital frequency declines with age, which reduces the risk of pregnancy. Coital frequency is also related to marriage duration; studies reveal that women married for 30 or more years have half the coital frequency rate of women married for four years or less. Reports of intercourse within the past month show a decline by age.
Coital frequency and acceptance of extramarital relations have been found to be culture-dependent. Kerin at al. show that oocyte production and fertilization rates decline with age. The quality of the uterus also declines with age. Navot et al. find a high pregnancy rate in women aged 40 years and older when oocytes are donated.
[J Biosoc Sci. 1994 Jul;26(3):349-68.]
- Reproductive potential in the older woman. [Fertil Steril. 1986]
- Fecundity and natural fertility in humans. [Oxf Rev Reprod Biol. 1989]
- Fertility in older women. [IPPF Med Bull. 1984]
- Age and infertility in a micronesian atoll population. [Hum Biol. 1993]
- Delaying childbearing: effect of age on fecundity and outcome of pregnancy. [BMJ. 1991]
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