When an Rh-negative woman becomes pregnant with an Rh-positive fetus (which can occur when the father's blood is Rh-positive), the pregnant woman's immune system produces antibodies that can destroy the fetus's blood. This antibody response is called Rh sensitization and occurs only if the fetus's blood mixes with the pregnant woman's, which can happen during birth.

To prevent Rh sensitization during pregnancy, you must have an Rh immune globulin injection if you are Rh negative. This is done during your pregnancy and after delivery to protect the fetus of a future pregnancy.