- and prenatal genetic testing in also on the upswing.

With more women having babies in their mid-to-late 30s--and even in their early 40s--prenatal testing and screening becomes more important. For those in whom the tests indicate a problem, however, tough decisions have to be made.

Genetic testing involves the simple and virtually painless procedure of amniocentesis--inserting a needle through the abdominal wall into the uterus to withdraw a small amount of amniotic fluid. Chromosomal patterns of cells in the fetus. Abnormal chromosomes can thus be detected and an accurate diagnosis of the problem made.

The risk of miscarriage in doing amniocentesis varies with the institution in which it is done, ranging from one in 200 to one in 300. At age 30, one in 952 pregnancies will produce a Down syndrome baby. At age 35, the risk is one in 378, and at 40, one in 106. The medical profession is thus in agreement that genetic testing should be done on all pregnant women 35 years of age and older.

Dr. David B. Weaver, director of clinical services in the department of medical genetics of the Indiana University School of Medicine, sees three current trends in prenatal diagnosis.

First, the number of conditions for which testing is available is rapidly increasing--about 800 different conditions at present, compared to 600 three or four years ago.

Second, more prenatal screening is being done with other kinds of tests that may indicate risk but do not give a diagnosis.

Third, doctors can now do things to deal with genetic problems before the baby is born that were unthinkable not long ago--such as opening the uterus surgically, operating on the fetus, and letting the pregnancy go on to term.