Reproductive risk is high during early adolescence, lowest in the 20s and rises gradually again after age 30. General health, diet, education, social class and medical care, however, all figure in a woman's chances of having ena normal pregnancy and offspring.

"Thus a 35-year-old educated middle-class woman, well nourished and in good health, who has access to first-rate medical care has more reason to expect an uncomplicated pregnancy and delivery than 20-year-old working-class woman who is overweight, prone to hypertension and without prenatal care," say Wellesley College researchers Pamela Daniels and Kathy Weingarten.

One of the greatest risks for women age 35 and older is of bearing a child with the chromosomal abnormality known as Down's syndrome, or mongolism. The chances of bearing a Down's syndrome child increases dramatically are 1 in 365 and by age 44 rise to 1 in 41.

Down's, however, can be detected during pregnancy by a test known as amniocentesis.

An increasing number of "elderly primogenitures" (women who bear children after age 30) are undergoing this medical precedure. It is best performed in a hospital at about 16 weeks of gestation generally under local anesthetic, according to the National Institute on Child Health and Development.

Aided by a sonogram (or "picture") to locate the position of the fetus and placenta, an obstetrician inserts a hollow needle through the abdominal wall into the uterus and extracts a small sample of the amniotic fluid.

Cells in the fluid are analyzed for chromosomal abnormalities. If abnormality is found, the choice may be made to abort the fetus. Abortions after the 15th week are performed in hospitals, using a solution to induce labor.

NICHD recommends the procedure to any pregnant woman 35 or older, if the woman has previously given birth to an abnormal child, if there is a family history of a genetically transmissible disease, or if there have been three or more miscarriages.

Within the realm of known defects, the diagnostic accuracy is more than 99 percent, and the risk of the procedure is less than one percent, according to a NICHD study. The cost can run more than $500, but some medical insurance policies cover all or part of the procedure.

And, as a side benefit, amniocentesis reveals the sex of the child.