Rh Factor

What Is Rh Factor?

The Rh factor is a type of protein that's usually found on red blood cells. When you have this protein, you’re considered Rh positive. About 85% of people are Rh-positive. The rest are Rh-negative. That means they don't have the protein. You get the protein from your parents at birth.

Who Gets the Rh Factor Test?

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

You usually get it in the first trimester unless you have vaginal bleeding. How many other tests you get depend on the results:

  • Rh positive: You won’t need another test.
  • Rh negative: You may get a test called an antibody screen to see if your blood has Rh antibodies. If you’re Rh negative and your baby is Rh positive, you might be likely to have a condition called RH factor incompatibility, which could be dangerous.

Rh Factor Incompatibility

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.

Antibody formation can be triggered by:

  • Complications early in pregnancy such as miscarriages, ectopic pregnancies, or terminations
  • Injury to the stomach during pregnancy
  • Bleeding during pregnancy
  • Amniocentesis and other tests that require fluids or cells to be withdrawn from the pregnant mother
  • Delivery, whether vaginal or cesarean

If you’re Rh negative and your baby is Rh positive, this means that your baby inherited the Rh gene from their Rh-positive dad.

How the Rh Factor Test Is Done

The Rh factor test is a simple blood test. It won’t harm you or your baby. The doctor will use a needle to take a small amount of blood from your arm.

Rh Factor Test Results

If you’re Rh-negative and your baby is Rh-positive, try not to worry. At around 28 weeks, the 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’ll need more shots of RhIG.

Let the doctor know if you have any spotting during pregnancy, especially if you’re Rh-negative. If you do, they may give you a shot of Rh immunoglobulin.

If you already have Rh antibodies, the drug won't work. Instead, your doctor will keep a close watch on your baby's health. Some babies need a blood transfusion after delivery. Others need one while they’re still in the womb.

WebMD Medical Reference Reviewed by Brunilda Nazario, MD on August 13, 2020

Sources

SOURCES:

American College of Obstetricians and Gynecologists: "The Rh Factor: How It Can Affect Your Pregnancy."

UptoDate: "Management of Rhesus (Rh) alloimmunization in pregnancy."

UptoDate: "Prevention of Rh(D) alloimmunization."

Cleveland Clinic: “Rh Factor”

University of Rochester: “Rh Typing.”

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