You Must Know Your Rh Factor if You Are Pregnant!

What is the Rh factor?

People with different blood types have specific proteins on the surfaces of their red blood cells (RBCs). There are four blood types — A, B, AB, and O. Each of the four blood types is further classified based on the presence of another protein on the surface of RBCs that indicates the Rh factor. If you carry this protein, you are Rh positive. If you don’t carry the protein, you are Rh negative. Most people — about 85% — are Rh positive.

How does a person get the Rh factor?

The Rh factor is inherited from parents to their children:

  • Mom Rh positive, Dad Rh positive -> baby Rh positive
  • Mom Rh positive, Dad Rh negative -> baby Rh positive or Rh negative
  • Mom Rh negative, Dad Rh positive -> baby Rh positive or Rh negative
  • Mom Rh negative, Dad Rh negative -> baby Rh negative

Can the Rh factor cause problems during pregnancy?

The Rh factor can cause problems if you (the mom) are Rh negative and your yet unborn baby is Rh positive. This is called “Rh incompatibility”. The problems of Rh incompatibility do not happen during the first pregnancy. They happen from the second pregnancy.

What happens if there is Rh incompatibility in the first pregnancy?

When your Rh-negative blood comes into contact with the baby’s Rh positive blood for the first time, it produces antibodies against the baby’s blood. Once this happens, your blood will be “Rh sensitized.”

How will my blood come into contact with my yet unborn baby’s blood?

During pregnancy, you and your baby do not share blood. However, a small amount of blood from the baby can cross the placenta into your system. This can sometimes happen during pregnancy, labor, or birth. It can also occur if an Rh-negative mother had any of the following procedures during pregnancy:

  • Amniocentesis
  • Chorionic villus sampling (CVS)
  • Bleeding during pregnancy
  • Manual rotation of a baby in a breech presentation before labor
  • Blunt trauma to the abdomen during pregnancy

Do problems often occur during pregnancy that cause Rh sensitization (during the first pregnancy)?

No. But you should receive a preventive treatment (see below) so that when you become pregnant again, your baby will have no Rh problems.

Can I still develop antibodies if my pregnancy is not carried to term?

Yes. It is possible to develop antibodies after a miscarriage, an ectopic pregnancy, or an induced abortion.

How does Rh sensitization affect the fetus during a second pregnancy or later?

Problems during a second pregnancy can occur when antibodies cross the placenta from you to the baby. The antibodies can causes hemolytic anemia and the baby can be damaged since he/she will not be able to get enough oxygen.

How can I find out if I have become Rh sensitized?

You can take a simple blood test, called an antibody screen, which will show if you have developed antibodies. If you are Rh negative and there is a possibility that your baby is Rh positive, your health care provider may request this test during your first trimester and again during week 28 of pregnancy.

Can Rh sensitization be prevented?

Yes. If you are Rh negative, you will be given a shot of Rh immunoglobulin (RhIg). The commercial name is RhoGAM. If you are Rh-negative and have not yet made antibodies against the Rh factor, RhIg can prevent fetal hemolytic anemia in a later pregnancy.

Can RhIg help me if I am already Rh sensitized?

RhIg is not helpful if you are already Rh sensitized.

When is RhIg given?

RhIg is given to Rh-negative women in the following situations:

  • At around the 28th week of pregnancy to prevent Rh sensitization for the rest of the pregnancy
  • Within 72 hours after the delivery of an Rh positive infant
  • After a miscarriage, abortion, or ectopic pregnancy
  • After amniocentesis or chorionic villus sampling

What if I am Rh sensitized and my fetus is Rh positive?

If you are Rh sensitized, you will be monitored during pregnancy to check the condition of your fetus. If tests show that your baby has severe anemia, it may be necessary to deliver your baby early (before 37 weeks of pregnancy) or give a blood transfusion while your baby is still in your uterus (through the umbilical cord). If the anemia is mild, your baby may be delivered at the normal time. After delivery, your baby may need a transfusion to replace the blood cells.

Diego Wyszynski
Dr. Diego Wyszynski is the Founder and CEO of Pregistry. He is an expert on the effects of medications and vaccines in pregnancy and lactation and an accomplished writer, having published 3 books with Oxford University Press and more than 70 articles in medical journals. In 2017, he was selected a TEDMED Research Scholar. Diego attended the University of Buenos Aires School of Medicine and Johns Hopkins School of Public Health.

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