Rhesus incompatibility causes a hemolytic anemia in newborns. When incompatible parents have a child, that child may be Rh positive. If his mother is Rh negative, the small quantity of fetal blood that passes the placental barrier can causes the mother to produce antibodies to the Rh factor (usually the D factor, the most common). This typically happens near or during childbirth, which is why the first child is safe most of the time. During subsequent pregnancies, these antibodies pass into the fetal blood, and if the baby is positive they attack and destroy its erythrocytes (red blood cells), causing severe anemia with cascading complications up to and including death. The problem can be forestalled by injecting the mother of anti-Rh immunoglobulin. This compound finds and destroys fetal Rh+ elements before the mother's immune system can detect them, preventing her from becoming sensitized (producing the antibodies). Treatment of an affected newborn ranges from observation (in very mild cases) to full blood transfusion in severe cases. Depending on how long the anemia persisted in utero, babies may be miscarried or may be born with severe deficits. At the time this episode was filmed, immunoglobulin therapy was relatively new. It is now routine for all pregnancies, given around 28 weeks.