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Rh Factor

By
WebMD Medical Reference
Reviewed by Nivin Todd, MD, FACOG

Who Gets the Test?

Every woman who's pregnant gets the Rh factor test. It's one of the first and most important tests you'll have.

What the Test Does

The Rh factor is a type of protein that's usually on blood cells. When you have this protein, you are considered Rh positive. About 85% of people are Rh-positive. The rest are Rh-negative -- they don't have the protein.

Normally, being Rh-negative has no risks. But during pregnancy, being Rh-negative can be a problem if your baby is Rh-positive. If your blood and your baby's blood mix, your body will start to make antibodies that can damage your baby's red blood cells. This could cause your baby to develop anemia and other problems.

How the Test Is Done

The Rh factor test is a simple blood test. It won’t harm you or your baby.

What to Know About Test Results

If you are Rh-negative and your baby is Rh-positive, try not to worry. At around 28 weeks, your doctor will give you a shot of Rh immunoglobulin (RhIG). This drug stops your body from making antibodies for the rest of your pregnancy. You may need a dose after delivery, too. If you get pregnant again later, you will need further shots of RhIG.

If you already have Rh antibodies, the drug won't work. Instead, your doctor will monitor your baby's health closely. Your baby may need a blood transfusion after delivery -- or sometimes while still in the womb.

How Often the Test Is Done During Your Pregnancy

Once.

Tests Similar to This One

An antibody screen, which checks an Rh-negative person for RH antibodies.

Reviewed on May 16, 2012

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