Rh factor is a protein that may be found on the surface of red blood cells. If you carry this protein, your blood is Rh positive. If you don't carry this protein, your blood is Rh negative.

Sometimes a mother with Rh-negative blood is pregnant with a baby that has Rh-positive blood. This can cause a problem if the baby's blood enters the mother's blood flow. The Rh-positive blood from the baby will make the mother's body create antibodies. This is called isoimmunization. The antibodies will attack any Rh-positive blood cells. This will not cause a problem for the mother. However, the antibodies can pass to the developing baby and destroy some of the baby's blood cells.

Fortunately, Rh incompatibility is often prevented with an immunization. If the condition is not prevented, the baby may need care.

A baby's Rh status is determined from the mother and father. If the mother is Rh negative and the father is Rh positive, the baby has at least a 50% chance of being Rh positive. However, Rh isoimmunization will only happen if the baby's Rh-positive blood enters the mother's blood flow. In most pregnancies, the mother's and baby's blood will not mix. The baby's blood may come into contact with the mother's blood flow during:

The mix in blood happens most often at the end of pregnancy. This means it is rarely a problem in a woman's first pregnancy. The mother's antibodies could affect a future pregnancy with a baby with Rh-positive blood even if the blood is not mixed.

A woman can also become sensitized to Rh-positive blood if she receives an incompatible blood transfusion.

Blood Flow to Fetus
Placenta Function
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