What does “Rh-negative” mean? — “Rh” is the name of a protein found on red blood cells. People who have this protein have an “Rh-positive” blood type. People who do not have this protein have an “Rh-negative” blood type. Having an Rh-positive or Rh-negative blood type does not affect your health. But problems can happen during pregnancy if a woman has Rh-negative blood and her baby has Rh-positive blood.

Fortunately, there is a medicine that can prevent these problems most of the time.

Women with Rh-positive blood do not have the problems discussed in this article.

How do I know my blood type? — Your doctor will do blood tests at your first prenatal visit. This includes a test to check your blood type.

What problems can happen when a woman has Rh-negative blood? — Certain problems can happen when a woman has Rh-negative blood and the baby in her uterus (womb) has Rh-positive blood.

Usually, during pregnancy, blood between the mother and baby does not mix. But sometimes it does, especially right after the baby is born. If a small amount of the baby's blood gets into the mother's blood vessels during delivery, the mother’s immune (infection-fighting) system makes proteins called “antibodies.” In the woman’s next pregnancy, these antibodies can cross the placenta and damage some of the next baby’s red blood cells. This can cause a condition called anemia, which is when a person has too few red blood cells. If this happens, the baby can make more red blood cells, but sometimes not enough to prevent anemia.

How do I know my baby’s blood type? — If both you and the baby’s father have Rh-negative blood, your baby will have Rh-negative blood. If your baby’s father has Rh-positive blood, your baby could have Rh-negative or Rh-positive blood. But if you have Rh-negative blood, the doctor will treat your baby as if he or she has Rh-positive blood. That way, any possible problems can be prevented.

What happens next? — If you are Rh-negative, your doctor will do blood tests during pregnancy to check whether you have antibodies in your blood.

If you do not have antibodies in your blood and your baby is (or could be) Rh-positive, your doctor will give you medicine. This medicine prevents your body from making antibodies that could attack your baby’s red blood cells. This can prevent anemia or other problems.

The medicine is called anti-D immune globulin (sample brand name: RhoGAM). It usually comes as a shot. You will get it during your third trimester.

After your baby is born, the doctor will do a blood test to check your baby’s blood type. If your baby is Rh-positive, your doctor will give you another dose of anti-D immune globulin soon after delivery. If your baby is Rh-negative, you do not need another dose.

Women who are Rh-negative might also get anti-D immune globulin at other times. Doctors give this medicine to Rh-negative women if they:

• Bleed during pregnancy
• Have a test during pregnancy called amniocentesis or chorionic villus sampling (CVS)
• Have a miscarriage, which is when a pregnancy ends on its own
• Have an abortion, which is when a woman ends a pregnancy
• Have an ectopic pregnancy, which is a pregnancy that grows in the wrong place in a woman’s body

What happens if I already have antibodies in my blood? — If you already have antibodies in your blood, anti-D immune globulin will not help. Your doctor will not give you this medicine.

But he or she will do different tests to monitor your baby during pregnancy. If anemia or other problems happen, he or she will talk with you about possible treatments.

What if I get pregnant again? — If you get pregnant again, you will need treatment again. Rh-negative women are treated with anti-D immune globulin during each pregnancy, as long as they don’t have antibodies in their blood