MED Risks, demands of pregnancy after 20

New York Times News Service

Biologically speaking, the ideal age at which to have a baby is between 18 and 20. That is when pregnancy is most likely to produce a healthy full-term child with the least risk to the health of the mother.

Today, however, millions of American women are postponing marriage and childbearing until their mid- to late 30s, the age at which our forebears became grandparents. Currently, in the United States one in five women had her first child after 35.

Unfortunately, one-third of women who wait until then to try to become pregnant are finding themselves infertile. With each passing year beyond 20, in fact, the likelihood of a successful pregnancy declines. Among women over 40, half will require medical assistance if they want to conceive.

Even then, conception does not guarantee a happy outcome for several reasons. Older women are more likely to suffer pregnancy complications; genetic abnormalities are more common in their fetuses; and the miscarriage rate rises as the fertility rate falls.


The effect of age on men's fertility is less clear. There is a small decline in sperm numbers and motility -- the ability of sperm to swim to an egg -- in men over 45. But even though these characteristics are less robust than in younger men, they usually remain within normal limits until about age 70. However, men over 40 are more likely to produce genetically damaged sperm and can transmit these defects, which cannot always be found through amniocentesis, to their offspring.

Writing in the journal Contemporary Ob/Gyn, Dr. Shirley Fong and Dr. Peter McGovern review what recent studies have revealed to explain why a woman's fertility declines steadily after age 20.

Even if a woman turns out to be capable of producing new eggs throughout her reproductive life span, older eggs remain a factor, the experts stated. An older egg is readily fertilized, but the resulting embryo has a harder time implanting in the uterus, where a placenta must form to provide a circulatory connection to the mother that nourishes and supports the fetus during its development. The placenta also produces hormones necessary for a successful pregnancy.

Older women have a higher incidence of uterine fibroids, benign tumors that can interfere with the attachment of the embryo to the uterine lining. Also, the lining of the uterus in older women appears to be less receptive to an embryo, possibly because their fertilized eggs produce an insufficient amount of the hormone progesterone, needed to maintain an early pregnancy until placenta production of progesterone takes over at about eight weeks. Implantation rates decline at age 30, and by age 40 are half that of a young woman. Older women receiving donor eggs are commonly treated with progesterone to improve implantation rates.

Older eggs, once fertilized, are more likely to divide abnormally, resulting in a genetically damaged embryo. Studies of embryos formed through in vitro fertilization showed that those of older women are five times more likely to have chromosomal disorders. Miscarriage occurs in 10 percent of women under 30 but in a third of women in their early 40s, with the greatest increase occurring after 35.

To reduce the rate of pregnancy loss among women undergoing IVF, preimplantation genetic analysis is now frequently performed on the early embryos formed from the eggs of older women. Sometimes all the embryos in an IVF cycle are found to be affected, which means none are suitable for implantation. However, this technique has significantly reduced miscarriage rates and increased the probability that a woman undergoing IVF will give birth to a genetically healthy baby.

Studies of infertile women who receive donor eggs show that problems with genetic abnormalities and miscarriage are related to the age of the egg. Most donor eggs come from women in their 20s, and the rates of miscarriage and chromosome errors match those of the donor, not the recipient. In fact, up to the late 40s, the Society of Reproductive Technology has reported, pregnancy rates are not affected by the age of a woman who conceives through egg donation. Not until age 50 and above are the rates of implantation, pregnancy and delivery significantly reduced in women receiving donor eggs.

Pregnancy is an assault on a woman's body, and the older a woman is, the less resilient her body is likely to be at meeting the many demands on it to support fetal growth and development.


Pregnancy in older women is associated with higher rates of three maternal health problems: gestational diabetes, pre-eclampsia (high blood pressure in pregnancy) and thrombophlebitis, the formation of blood clots. These problems can threaten the well-being and even the life of the mother and her fetus.

Jane E. Brody is a New York Times staff writer and columnist who specializes in health and medical reporting.

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