Prenatal Antibody Testing

When you're a mom-to-be, one of the prenatal tests you may get is an antibody test or antibody screening. It looks for certain antibodies, special proteins made by your immune system, in your blood.

You might have these antibodies if you've gotten blood from a donor or given birth before. There's also a chance your body could make them when your and your baby's blood types don't match.

Some of these antibodies can be passed from you into your baby's bloodstream, where they could do harm. Antibody testing lets you and your doctor know if you have them so you can take steps to protect your growing baby.

Why You Get Tested

Your immune system makes antibodies to fight things it sees as "not you." Most of the time, that's great because antibodies usually target germs. And when you're pregnant, your immune system takes care of your baby, too. But if your red blood cells are different from your baby's, that may cause problems.

By far, the most common one is related to the + or - part of your blood type, called the Rh factor. Many people are Rh-positive, which means they have the Rh protein on their red blood cells. Rh-negative people don't. So they'll make antibodies to attack any Rh-positive blood cells that get into their body.

If you're Rh-negative and your baby is Rh-positive, your blood might have Rh antibodies that could spread to your baby's blood, where they'd attack and destroy your baby's red blood cells. This can cause a type of anemia that's very serious and could be fatal.

Your body might have made other antibodies that could attack your baby's red blood cells, too.

How It's Done

You should get your blood type checked early in your pregnancy, perhaps at your first prenatal visit. If you're Rh-negative, then you should have the antibody test during the first 3 months that you're pregnant. (If you're Rh-positive, your doctor may still want to do an antibody test in your first trimester.)

A technician uses a needle to take a sample of blood from a vein in your hand or arm. You may feel a small skin prick and have a little bleeding or bruising where the needle goes in.

Then they'll send the sample to a lab to run an indirect Coombs test, which checks for red blood cell antibodies.

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What the Results Mean

A negative antibody test tells you that you don't have harmful antibodies in your blood. If you're also Rh-positive, you can safely carry a baby with either a + or - blood type. Relax and enjoy being pregnant!

If the test was negative and you're Rh-negative -- but there's a chance your baby is Rh-positive (because the father is) -- you'll need another test about 28 weeks into your pregnancy. If it's negative again, your doctor will probably give you a shot of medicine called Rho(D) immune globulin (RhoGAM, RhIG, WinRho) to stop your immune system from making Rh antibodies.

These antibodies might not cause trouble for your first baby, but the shot will also help prevent trouble if you get pregnant again.

A positive test means you already have antibodies in your blood. If they're Rh antibodies, the shot won't help. Your doctor will watch you and your baby closely. If there are problems while you're pregnant, your baby may need to be born early or get a blood transfusion through the umbilical cord.

WebMD Medical Reference Reviewed by Neha Pathak, MD on November 23, 2017

Sources

SOURCES:

LabTestsOnline.org: "Pregnancy & Prenatal Testing," "RBC Antibody Screen."

American Congress of Obstetricians and Gynecologists: "The Rh Factor: How It Can Affect Your Pregnancy," "Routine Tests During Pregnancy."

Mayo Clinic: "Rh factor blood test: Why it's done."

University of Rochester Medical Center: "Rh Disease."

© 2017 WebMD, LLC. All rights reserved.

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