One of the first tests a pregnant woman should expect is a blood-type test. This test checks her blood type and Rh factor. Her Rh factor may play a role in her baby's health, so it's important to know this information early in pregnancy.

What's an Rh Factor?

An Rh factor is a protein found on some red blood cells (RBCs). Not everyone carries this protein, though most do. They are Rh-positive. People who don't carry the protein are Rh-negative.

What Is Rh Incompatibility?

When a mother-to-be and father-to-be are not both positive or negative for Rh factor, it's called Rh incompatibility.

For example:

• If a woman who is Rh negative and a man who is Rh positive conceive a baby, the fetus may have Rh-positive blood, inherited from the father. (About half of the children born to an Rh-negative mother and Rh-positive father will be Rh-positive.)

Rh incompatibility usually isn't a problem if it's the mother's first pregnancy. That's because the baby's blood does not normally enter the mother's circulatory system during the pregnancy.

During the birth, though, the mother's and baby's blood can mix. If this happens, the mother's body recognizes the Rh protein as a foreign substance. It then might begin making antibodies (proteins that act as protectors if foreign cells enter the body) against the Rh protein.

Rh-negative pregnant women can be exposed to the Rh protein that might cause antibody production in other ways too. These include:

• blood transfusions with Rh-positive blood 
• miscarriage
• ectopic pregnancy

When Is a Baby at Risk?

Rh antibodies are harmless until the mother's second or later pregnancies. If she is ever carrying another Rh-positive child, her Rh antibodies will recognize the Rh proteins on the surface of the baby's blood cells as foreign. Her antibodies will pass into the baby's bloodstream and attack those cells.

This can make the baby's red blood cells swell and rupture. This is known as hemolytic or Rh disease of the newborn. It can make a baby's blood count get very low.

How Is Rh Incompatibility Treated?

If a pregnant woman has the potential to develop Rh incompatibility, doctors give her a series of two Rh immune-globulin shots during her first pregnancy. She'll get:

• the first shot around the 28th week of pregnancy
• the second shot within 72 hours of giving birth

Rh immune-globulin acts like a vaccine. It prevents the mother's body from making any Rh antibodies that could cause serious health problems in the newborn or affect a future pregnancy.

A woman also might get a dose of Rh immune-globulin if she has a miscarriage, an amniocentesis, or any bleeding during pregnancy.

If a doctor finds that a woman has already developed Rh antibodies, her pregnancy will be closely watched to make sure that those levels are not too high.

In rare cases, if the incompatibility is severe and a baby is in danger, the baby can get special blood transfusions called exchange transfusions either before birth (intrauterine fetal transfusions) or after delivery. Exchange transfusions replace the baby's blood with blood with Rh-negative blood cells. This stabilizes the level of red blood cells and minimizes damage from Rh antibodies already in the baby's bloodstream.

Thanks to the success rate of Rh immune-globulin shots, exchange transfusions in Rh-incompatible pregnancies are rarely needed in the United States.

What Else Should I Know?

If you're not sure what your Rh factor is and think you're pregnant, it's important to start regular prenatal care as soon as possible — including blood-type testing. With early detection and treatment of Rh incompatibility, you can focus on more important things — like welcoming a new, healthy baby.