A pregant woman gets her blood tested by a doctor. Her finger is pricked for the doctor to obtain her blood.
A mom with Rh-negative blood should consult her doctor about a RhoGAM shot during pregnancy. (For Spectrum Health Beat)

I remember doing a science experiment with our older children—an at-home medical test where we could poke our fingers to learn about our blood types.

The children enjoyed learning about it. My husband is A-positive and I’m AB-positive, which is a rarer type.

According to the American Red Cross, Type O-positive is the most common blood type among all races and ethnicities in the U.S., followed by A-positive. AB-negative is the rarest of blood types. After that, the distribution of blood type can vary by race and ethnicity.

The American Red Cross has a comprehensive chart on the various blood types as they appear in the population.

You may have heard that AB is the universal receiver and O is the universal donor. The A, B, AB and O types relate to the types of antigens in the blood, while the positive and negative indicates the presence or absence of the Rh factor, a type of protein on red blood cells.

It surprised me when our daughter, who is expecting a baby in May, mentioned to me she is Type AB-negative. We had tested her blood type but not her Rh factor.

She has a pretty rare blood type. Only about 1 percent of the population is AB-negative.

While researching the science behind this, I learned that my husband and I both have secondary Rh factors that are negative, which she inherited. Thus, my daughter’s AB-negative blood.

Now, you may be wondering what this has to do with pregnancy. It all relates to something called Rh incompatibility.

Rh incompatibility

The American Congress of Obstetricians and Gynecologists explains that the Rh factor can sometimes cause problems during pregnancy, primarily when the mom is Rh negative and her fetus is Rh positive.

This is called Rh incompatibility.

“These problems usually do not occur in a first pregnancy, but they can occur in a later pregnancy,” the American Congress of Obstetricians and Gynecologists website explains. “If you are (Rh) positive, your red blood cells have a protein on them that Rh-negative people don’t have.”

The mom’s blood and the baby’s blood don’t really intermingle until the baby is born.

At birth, if there is Rh incompatibility—an Rh-negative mom and an Rh-positive baby—the mom’s blood will create antibodies because the baby’s blood is recognized as a foreign substance.

The National Heart, Lung and Blood Institute explains how this is not a problem during a mother’s first birth, as the baby is typically born before mom’s antibodies can develop.

In subsequent births, however, it can be a problem.

If the Rh antibodies cross into the placenta during a later birth and attack the baby’s red blood cells, it may lead to hemolytic anemia in the baby, which can make it hard for the baby to get oxygen.

We can give the mom a shot of RhoGAM, which keeps her body from detecting the baby’s blood as a foreign substance. This prevents her Rh antibodies from attacking the baby’s red blood cells.

The takeaway: If you have a negative Rh factor, you will need an injection of RhoGAM during pregnancy. It is made from blood plasma and given as an injection. The FDA approved it for use in 1968.

Here is when an Rh-negative mom should have RhoGAM:

  • At about the 28th week of pregnancy to help prevent Rh sensitization for the rest of the pregnancy.
  • Within 72 hours after delivery of an Rh-positive infant. Your baby’s blood will be tested, and if they are negative you will not need the RhoGAM.
  • After a miscarriage, abortion or ectopic pregnancy.
  • After amniocentesis or chorionic villus sampling, or uterine bleeding.

The RhoGAM injection contains a very small amount of the Rh-positive blood antibodies to help the mom’s body “think” it has already reacted to the foreign blood type.