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.

Rh sensitization can occur during pregnancy if you are Rh-negative and pregnant with a developing 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.

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.

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

All pregnant women get a blood test at their first prenatal visit during early pregnancy. This test will show if you have Rh-negative blood and if you are Rh-sensitized.

If you have Rh-negative blood but are not sensitized:

• The blood test may be repeated between 24 and 28 weeks of pregnancy. If the test still shows that you are not sensitized, you probably will not need another antibody test until delivery. (You might need to have the test again if you have an amniocentesis, if your pregnancy goes beyond 40 weeks, or if you have a problem such as placenta abruptio, which could cause bleeding in the uterus.)
• Your baby will have a blood test at birth. If the newborn has Rh-positive blood, you will have an antibody test to see if you were sensitized during late pregnancy or childbirth.

If you are Rh-sensitized, your doctor will watch your pregnancy carefully. You may have:

• Regular blood tests, to check the level of antibodies in your blood.
• Doppler ultrasound, to check blood flow to the baby's brain. This can show anemia and how severe it is.
• Amniocentesis after 15 weeks, to check the baby's blood type and Rh factor and to look for problems