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Blood types include A, B, AB, and O. Each of these blood types can be either Rh negative or Rh positive (A-, A+, B-, B+, AB-, AB+, O-, O+). Only about 15% of people are Rh negative. This means that their red blood cells do not have a particular antigen on the surface. Antigens are what your body uses to recognize something as foreign. If your own blood cells have this antigen on them (i.e if you are Rh positive), your body will recognize this antigen as belonging to you and not create antibodies to attack that antigen. However, if you do not have this antigen on your own red blood cells (i.e you are Rh negative), any time your body encounters this antigen it will treat it as foreign and create antibodies to attack it. One way your body might encounter this antigen is if you are given a blood transfusion from a donor who is Rh positive. The other way your body might encounter this antigen is if your baby inherits Rh positive blood from the father.

If you are Rh negative and your baby is Rh positive, while you are pregnant your immune system will create antibodies to the antigen that is present on the baby’s red blood cells. Usually you do not produce enough antibodies early enough to cause a problem in a first pregnancy, however in any subsequent pregnancy the antibodies are present from before and are ready to mount a response. Without intervention, these antibodies identify the Rh positive fetus’s red blood cells as “foreign” and begin to attack them. This causes anemia in the fetus as the red blood cells are destroyed, and eventually this can result in death of the fetus.

Fortunately we have a way to prevent this from happening. We can administer Rh immune globulin (WinRho) which prevents the formation of antibodies. WinRho is given to pregnant women who are Rh negative at 28 weeks gestation, and earlier if there is threatened miscarriage, bleeding, or trauma. It is given again after delivery if the infant is confirmed to be Rh positive.