Rh Factor (Twins)

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 either of your babies is Rh-positive. If your blood and your Rh-negative babies' blood mix, your body will start to make antibodies that can damage that babies' red blood cells. This could cause your Rh-negative babies 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 twins.

What to Know About Test Results

If you are Rh-negative and either or both of your babies are 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. Your doctor may choose to give you a dose when you have any vaginal bleeding or spotting during pregnancy as well. Check with your doctor if you do have any bleeding or spotting, especially if you are Rh-negative. 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 the health of your babies closely. Your babies may need a blood transfusion after delivery -- or sometimes while still in the womb.

How Often the Test Is Done During Your Pregnancy


Tests Similar to This One

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

WebMD Medical Reference Reviewed by Traci C. Johnson, MD on January 17, 2019



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."

© 2019 WebMD, LLC. All rights reserved.

Get Pregnancy & Parenting Tips In Your Inbox

Doctor-approved information to keep you and your family healthy and happy.

By clicking Subscribe, I agree to the WebMD Terms & Conditions & Privacy Policy and understand that I may opt out of WebMD subscriptions at any time.