Rh incompatibility occurs when the mother's blood type is Rh-negative (Rh-) and her unborn baby's blood type is Rh-positive (Rh+). If the mother is Rh+, or if both parents are Rh-, there is no reason to worry about Rh incompatibility.
If the baby's blood is Rh+ and gets into the mother's Rh- bloodstream, her body will produce antibodies. These antibodies could pass back through the placenta and harm the developing baby's red blood cells. This can cause mild to serious anemia in the unborn baby.
This test determines the amount of blood that has been exchanged between the mother and fetus. All Rh- pregnant women should get this test if they have bleeding or a risk of bleeding during the pregnancy.
In a woman whose blood is Rh incompatible with her infant, this test helps determine how much Rh immune globulin (RhoGAM) she must receive to prevent her body from producing abnormal proteins that attack the unborn baby in future pregnancies.