If you’re a pregnant woman with Rh-negative blood, and the baby’s father has Rh-positive blood (or unknown type) your doctor may prescribe an Rho(D) immune globulin  injection for you called RhoGAM. This injection can prevent a disease called Rh-induced hemolytic disease which can harm the fetus you’re carrying and any other fetus you may conceive. The disease’s effects on a fetus or newborn can be mild or serious, ranging from mild anemia and jaundice to mental retardation and death.

Rh-induced hemolytic disease is caused by the incompatibility between your Rh-negative blood and a fetus’s Rh-positive blood.During pregnancy, fetal blood cells can enter your bloodstream through the placenta. When this happens, your immune system will treat the fetal Rh-positive blood cells as if they are harmful invaders and makes antibodies against them. If these antibodies pass back into your fetus through the placenta, they begin to destroy the fetus’s blood cells.

A Rho(D) immune globulin injection stops your body from making antibodies. This protects your baby against Rh-induced hemolytic disease. Your first injection must happen during pregnancy, before your body has begun to make antibodies. After your baby is born and had a blood test, you will need a second injection if your baby turns out to have Rh-positive blood.

For more details, see The RH Factor from ACOG.