What is Rh sensitization during pregnancy?

If you are Rh-negative, your red blood cells do not have a marker called Rh factor on them. Rh-positive blood does have this marker. If your blood mixes with Rh-positive blood, your immune system will react to the Rh factor by making antibodies to destroy it. This immune system response is called Rh sensitization.

What causes Rh sensitization during pregnancy?

Rh sensitization can occur during pregnancy if you are Rh-negative and pregnant with an unborn baby (fetus) who has Rh-positive blood. In most cases, your blood will not mix with your baby’s blood until delivery. It takes a while to make antibodies that can affect the baby, so during your first pregnancy, the baby probably would not be affected.

But if you get pregnant again with an Rh-positive baby, the antibodies already in your blood could attack the baby’s red blood cells. This can cause the baby to have anemia, jaundice, or more serious problems. This is called Rh disease. The problems will tend to get worse with each Rh-positive pregnancy you have.

During your first pregnancy, your baby could be at risk for Rh disease if you were sensitized before or during pregnancy. This can happen if:

• You had a previous miscarriage, abortion, or ectopic pregnancy and you did not receive Rh immune globulin to prevent sensitization.
• You had a serious injury to your belly during pregnancy.
• You had a medical test such as an amniocentesis or chorionic villus sampling while you were pregnant, and you did not receive Rh immune globulin. These tests could let your blood and your baby’s blood mix.

Rh sensitization is one reason it’s important to see your doctor in the first trimester of pregnancy. It doesn't cause any warning symptoms, and a blood test is the only way to know you have it or are at risk for it.

• If you are at risk, Rh sensitization can almost always be prevented.
• If you are already sensitized, treatment can help protect your baby.

Who gets Rh sensitization during pregnancy?

Rh sensitization during pregnancy can only happen if a woman has Rh-negative blood and only if her unborn baby has Rh-positive blood.

• If the mother is Rh-negative and the father is Rh-positive, there is a good chance the baby will have Rh-positive blood. Rh sensitization can occur.
• If both parents have Rh-negative blood, the baby will have Rh-negative blood. Since the mother’s blood and the baby’s blood match, sensitization will not occur.

If you have Rh-negative blood, your doctor will probably treat you as though the baby’s blood is Rh-positive no matter what the father’s blood type is, just to be on the safe side.

How is Rh sensitization diagnosed?