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What Is Rh Incompatibility?

Rh incompatibility is a condition that occurs during pregnancy if a woman has Rh-negative blood and her baby has Rh-positive blood.

"Rh-negative" and "Rh-positive" refer to whether your blood has Rh factor. Rh factor is a protein on red blood cells. If you have Rh factor, you're Rh-positive. If you don't have it, you're Rh-negative. Rh factor is inherited (passed from parents to children through the genes). Most people are Rh-positive.

Whether you have Rh factor doesn't affect your general health. However, it can cause problems during pregnancy.


When you're pregnant, blood from your baby can cross into your bloodstream, especially during delivery. If you're Rh-negative and your baby is Rh-positive, your body will react to the baby's blood as a foreign substance.

Your body will create antibodies (proteins) against the baby's Rh-positive blood. These antibodies usually don't cause problems during a first pregnancy. This is because the baby often is born before many of the antibodies develop.

However, the antibodies stay in your body once they have formed. Thus, Rh incompatibility is more likely to cause problems in second or later pregnancies (if the baby is Rh-positive).

The Rh antibodies can cross the placenta and attack the baby's red blood cells. This can lead to hemolytic anemia (HEE-moh-lit-ick uh-NEE-me-uh) in the baby.

Hemolytic anemia is a condition in which red blood cells are destroyed faster than the body can replace them. Red blood cells carry oxygen to all parts of the body.

Without enough red blood cells, your baby won't get enough oxygen. This can lead to serious problems. Severe hemolytic anemia may even be fatal to the child.


With prompt and proper prenatal care and screening, you can prevent the problems of Rh incompatibility. Screening tests allow your doctor to find out early in your pregnancy whether you're at risk for the condition.

If you're at risk, your doctor will carefully check on you and your baby throughout your pregnancy and prescribe treatment as needed.

Injections of a medicine called Rh immune globulin can keep your body from making Rh antibodies. This medicine helps prevent the problems of Rh incompatibility. If you're Rh-negative, you'll need this medicine every time you have a baby with Rh-positive blood.

Other events also can expose you to Rh-positive blood, which could affect a pregnancy. Examples include a miscarriage or blood transfusion. If you're treated with Rh immune globulin right after these events, you may be able to avoid Rh incompatibility during your next pregnancy.

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Last Updated: January 1, 2011