V.

Detection

Scientists have developed several tests for defective genes and for fetal disorders. For some hereditary conditions such as Tay-Sachs disease and sickle-cell anemia, tests are available to detect the defective gene in the healthy parents.

Many diseases can be diagnosed in the fetus by drawing a sample of the amniotic fluid surrounding the fetus and culturing fetal cells for analysis or testing the fluid itself (see Amniocentesis). Another method of obtaining fetal cells for study early in pregnancy is called chorionic villus sampling. The fetal cells can be tested for the presence of a particular function, or their DNA (see Nucleic Acids) can be scanned for evidence of a genetic disorder. Also, a sample of fetal blood for testing may be obtained directly from the umbilical cord, as may be necessary in cases where DNA diagnosis is not possible.

Some tests for birth defects can be performed on the mother without being as invasive as amniocentesis, chorionic villus sampling, or fetal blood sampling. One of these methods is ultrasonography (see Ultrasound), which can reveal a number of abnormalities, as well as an accurate determination of fetal age, the presence of multiple pregnancies, and even fetal sex. Elevated levels of alphafetoprotein in a pregnant woman’s blood may indicate the presence of spina bifida; depleted levels sometimes indicate the presence of Down syndrome. Unusual concentrations of two other substances in maternal serum—unconjugated estriol and human chorionic gonadotropin—have also been found when the fetus has Down syndrome.

Many structural birth defects that result from arrested or incomplete development are amenable to corrective surgery (see Plastic Surgery). These include cleft lip and palate, failure of development of parts of the digestive tract, and openings between the chambers of the heart. The treatment of hereditary diseases with the techniques of genetic engineering is a novel approach, with great promise for the future, that has already reached the stage of clinical trials.