While advances in examining the fetus have been used to diagnose extremely severe defects, these same techniques have opened up possibilities for treating certain fetal disorders. The best-known example is treatment of hemolytic disease of the newborn due to Rh incompatibility.

This blood disorder occurs if the woman’s blood is Rh negative and she is carrying a baby whose blood is Rh positive and she has previously been sensitized to Rh-positive blood. The woman can be sensitized through miscarriage, elective abortion, amniocen­tesis, a previous full-term pregnancy, or a blood transfusion of Rh-positive blood. In each situation except the last, Rh-positive blood has passed from the fetus to the Rh-negative woman. During subse­quent pregnancies, the woman produces antibodies directed against the fetus’s Rh-positive blood cells because her immune system recognizes the Rh-positive blood cells as “foreign.” The maternal antibodies, resulting in the development of profound anemia, destroy the fetal blood cells.

If the fetus is found to be severely anemic before he or she is mature enough to be delivered safely (about 30 weeks’ gestation), one or more blood transfusions are given via a fetoscope (fiberoptic viewing tube). The blood is transfused into one of the blood vessels in the umbilical cord. Alternatively, blood may be injected directly into the heart of the fetus via a needle passed through the mother’s abdominal wall and into the uterus. This obstetrical technique is called intracardiac transfusion and relies on accurate visualization of the fetus with ultrasound scanning. Although these measures carry some risk of inducing a miscarriage, the risk is usually justified because the ultimate outlook is excellent if the fetus can be kept alive long enough for birth to be feasible.

Sometimes, ultrasound tests show the presence of a defect in the fetus’s urinary system, causing obstruc­tion and pressure on the kidneys. In these circum­stances, a fetoscope may be used to perform a simple operation to relieve the pressure and thus limit any damage to the kidneys.