Rh Factor

Medically Reviewed by Dan Brennan, MD on August 09, 2022
3 min read

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.

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.

Most of the time, 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 is known as Rh sensitization.

Rh sensitization is unlikely to harm the first Rh-positive baby that you carry, because you’ll rarely come into contact with your baby’s blood until labor and delivery, meaning the antibodies won’t be created until after birth.

But once you are Rh sensitized, the Rh antibodies stay in your system. If you get pregnant with another Rh-positive baby, your Rh antibodies will attack this baby’s blood while they’re growing inside you. This can cause Rh disease in your baby.

Rh disease causes hemolytic anemia, which destroys red blood cells faster than the body can create them. It can cause serious illness or even death for your baby.

Even though you and your baby don’t share blood, some of your blood can mix for various reasons. Most of the time, it happens during labor and delivery, but it can also happen:

  • During amniocentesis, a test that uses a needle to take cells from the fluid that surrounds your baby inside the womb
  • During chorionic villus sampling (CVS), a test that uses a long needle to take cells from the placenta (tissue within your womb that you use to nourish your baby)
  • If you have vaginal bleeding when you’re pregnant
  • If you have an injury to your belly while you’re pregnant
  • If your baby is breech (feet-first) and your doctor tries to turn them around by pressing on your belly
  • If you have a miscarriage, an ectopic pregnancy (a life-threatening problem that happens when a baby starts to grow outside the womb), or an abortion

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.

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.

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.

Show Sources


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

UpToDate: "Management of Rhesus (Rh) alloimmunization in pregnancy," "Prevention of Rh(D) alloimmunization."

Cleveland Clinic: “Rh Factor.”

University of Rochester: “Rh Typing.”

Nemours Foundation: “Rh incompatibility.”

National Heart, Lung, and Blood Institute: “How is Rh incompatibility treated?” “How is Rh incompatibility diagnosed?” “What is Rh incompatibility?” “Who is at risk for Rh incompatibility?”

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