The risks of reproductive impairment in the later years of childbearing.

The evidence for impaired fertility of U.S. women after their mid-30s is reviewed in the context of historical studies on age-related fertility, and the factors determining subfecundity, particularly coital inability, conceptive failure and pregnancy loss. For comparison, early American women produced 7 children, falling to a low of 1.74 children in 1976. The factors affecting fertility are known as the Davis-Blake intermediate variables. Those in the 2nd category, mate exposure, affect middle-aged women because most childbearing is done by married women, and educated women over 30-34 begin to have difficulty finding husbands. Fecundity ranges from 0-30 children, and averages 15 in optimal conditions. Fecundity falls with age, but subfecundity may occur in addition due to factors such as pelvic inflammatory disease or complications of childbirth. The 1982 National Survey of Family Growth, and other studies find fecundity falling moderately with time but significantly after age 35. Historical studies show that fecundity falls as marital duration lengthens, owing to subfecundity and also to decreased coital frequency. Recent work suggests a high rate of sexual dysfunction, often underreported, ranging from 4% permanent impotence in men 40 to 40% of men and 63% of women. Conceptive failure is estimated at 8.5% of married women aged 15-44, rising sharply after age 35. Pregnancy loss is thought to be the most common cause of subfecundity due to aging. Sensitive tests show that 75% of all fertilized ova and 31% of all implanted embryos are lost, but only 25% of women are aware of pregnancy loss. The bottom line is that U.S. women who postpone childbearing until their 30s will have fewer children than they originally desired, due to a wide variety of factors.