Without question genetic counseling has brought joy to hundreds of parents who, without it, would not have risked having children. Genetic counseling has helped other couples decide not to have children when the possibility of a birth defect turned out to be high. With the increasing availability of counseling centers (there are now 300 in the United States), counseling should help reduce the number of infants born with birth defects, now a quarter million each year. Yet if a baby is already on the way, genetic counseling may make use not just of pedigree studies and chromosome tests (karyotypes) of the inquiring couples, but also of prenatal diagnosis. Prenatal diagnosis allows advance detection of birth defects by withdrawal and examination of sample fetal cells from the mother-to-be's uterus. Contrary to popular opinion, it is far from routine clinical procedure. True, amniocentesis-the withdrawal by needle of some of the fluid that bathes the fetus-has been applied late in pregnancy to detect to what extent an Rh-positive baby's blood has been destroyed by antibodies from its Rhnegative mother, so that a transfusion can be given to the baby. Tens of thousands of women have undergone this procedure in the past decade, and today it is considered common medical practice in many hospitals. But experience with amniocentesis early in pregnancy, necessary for intrauterine diagnosis of birth defects, is limited to only several hundred patients, all in the past four years. Although any immediate serious risk to the fetus appears to be small (one percent), the long-term risks of the tap to the fetus are unknown. Taps must also be repeated 10 percent of the time to arrive at a diagnosis. Moreover, as Dr. Orlando Miller of the Columbia University College of Physicians and Surgeons points out, blood cells from the fetus pass into the maternal blood in 10 percent of the cases. This passage could pose a problem by sensitizing Rh-negative mothers with Rh-positive babies. Hence Dr. Henry Nadler of Northwestern University, and other experts at early-pregnancy amniocentesis, urge physicians to move cautiously before tapping for prenatal diagnosis and not to be swept into the procedure by public demand for it. Even if the fetus is not damaged, the precious few fetal cells withdrawn in the first 14 to 16 weeks of pregnancy must be cultured until there are enough of them to diagnose metabolic deficiencies or chromosome defects. It can easily require four weeks to grow enough cells to determine sex-linked birth defects such as hemophilia or a type of muscular dystrophy, and four to eight weeks to determine severe metabolic diseases such as Tay-Sach's disease (which causes progressive degeneration of the nervous system), Pompe's disease (which results in an enlarged liver and spleen and possibly mental retardation) or lysosomal acid phosphatase deficiency (which leads to vomiting, lethargy and death). If after this delay, cell culturing and diagnosis indicate the advisability of a therapeutic abortion and the parents desire one, it may by then be too late. Also, doctors have to be careful that the cells under study are really from the fetus and not from the mother. Knotty ethical questions about prenatal diagnosis have also been raised. Some of them were discussed by prenatal diagnosis specialists at a National Fetal taps and tests are helping reduce birth defects, but they are far from the final answer