Overview

One of the first tests you may have when you’re pregnant is a blood test to check your blood type and Rhesus (REE-suhs) factor, or Rh factor. Your Rh factor may play a role in your baby's health, so it's important to know this information early in pregnancy.

What's an Rh Factor and Rh Incompatibility?

An Rh factor is a protein found on some people's red blood cells. Most people have this protein and they’re called Rh-positive. People who don't have the protein are called Rh-negative.

When you’re Rh-negative and your baby is Rh-positive, it's called Rh incompatibility (in-kem-pah-tuh-BIH-luh-tee).

Top Things to Know

• Rh incompatibility is when the blood of a fetus has a protein called Rh factor and the mother’s blood doesn’t.
• This can cause anemia, making a baby’s level of red blood cells very low.
• Getting a medicine called RhoGAM, or Rho(D) immune globulin, can help prevent anemia and other problems.
• Rh incompatibility usually isn’t a problem in the first pregnancy but it can be in later ones.

What Happens With Rh Incompatibility?

If you’re Rh-negative and your baby’s father is Rh-positive, the baby may have Rh-positive blood, inherited from the father. (About half of children born to a Rh-negative mother and a Rh-positive father will be Rh-positive.)

Rh incompatibility usually isn't a problem if it's your first pregnancy. That's because the baby's blood doesn’t normally enter your circulatory system (the way blood moves through the body) while you’re pregnant.

But sometimes the mother's and baby's blood can mix. This can happen due to delivery, tests like amniocentesis, vaginal bleeding, a doctor moving a baby from feet- or bottom-first to head-first before birth, an injury to the mother’s belly, a miscarriage, or an abortion. If the blood mixes, the mother's body sees the Rh protein in the baby’s blood as a foreign substance or something that shouldn’t be there. The mother’s body then starts making antibodies (proteins that act as protectors) against the Rh protein.

If you’re Rh-positive and your baby’s father is Rh-negative, it’s OK if your baby is Rh-negative. Your Rh-positive blood won’t make antibodies.

When Is a Baby at Risk?

Rh antibodies are harmless until the second pregnancy or a later one. If a mother ever carries another Rh-positive child, her Rh antibodies will see 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 burst — it’s known as hemolytic or Rh disease of the newborn. It can make a baby's red blood cell level get very low, which can be harmful. Rh incompatibility can also cause jaundice (yellowing of the skin and white of the eyes). Severe Rh incompatibility can lead to problems with organs like the brain, liver, and heart.

Rh incompatibility doesn’t affect the mother’s health.

Diagnosis

When Is Rh Factor Tested?

You’ll likely have an Rh factor blood test in the first trimester of pregnancy at your first prenatal appointment. If you have Rh incompatibility, your doctor will create a care plan just for you and your baby.

How Is Rh Incompatibility Treated?

Rh Immune Globulin

Rh incompatibility can be treated with a medicine called RhoGAM, or Rho(D) immune globulin, which acts like a vaccine. It prevents the body from making any Rh antibodies that could cause serious health problems for the baby or affect a future pregnancy.

If there’s a chance you’ll develop Rh incompatibility, you’ll get two shots of Rh immune globulin during each pregnancy. You’ll have:

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

You also might get a dose of Rh immune globulin if you have a miscarriage, an amniocentesis test, or any vaginal bleeding or injuries to the belly during pregnancy.

If doctors find that your body has already made Rh antibodies, they’ll watch your pregnancy closely with tests called ultrasounds.

Blood Transfusion

In rare cases, if the incompatibility is severe and a baby is in danger, the child might need an intrauterine transfusion (a blood transfusion while in the womb). After delivery, a baby might need a blood transfusion called an exchange transfusion.

Exchange transfusions replace the baby's blood with blood that has Rh-negative blood cells. This keeps the level of red blood cells steady. It also lessens damage from Rh antibodies that are already in the baby's bloodstream.

Thanks to the success rate of Rho(D) immune globulin shots, exchange transfusions are rarely needed for Rh incompatibility 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. If you have Rh incompatibility, it means you have a “high-risk” pregnancy, so be sure to work with your doctor to get treated. That way, you can focus on getting ready for your baby.

Other Common Questions

What should I know about medical care during pregnancy?

The sooner care starts in pregnancy, the better for the health of both you and your baby. Find out what prenatal care involves.

What might surprise me about being pregnant?

You may think you’ve planned for everything in pregnancy, but you might not expect stuff like skin changes and too-tight shoes. Learn about surprising pregnancy changes.

How can I stay healthy during pregnancy?

Now that you're pregnant, taking care of yourself has never been more important. Get tips for a healthy pregnancy.