iStock/ThinkstockBy DR. JENNIFER ASHTON, ABC News Senior Medical Contributor

Many of us learned about the different blood types in school. But you may not have learned about your blood’s Rh factor — and, specifically, how it can affect a pregnancy.

Rh, or rhesus factor, is a protein that can appear on the surface of red blood cells, and it’s passed down through your parents’ genes. Some people are Rh positive, while others are Rh negative.

The Rh factor can cause problems in a pregnancy if there’s incompatibility between the mother and baby. So here’s what you need to know:

• If you’re Rh negative and carrying a fetus who is Rh positive, your body’s immune system can start to attack the fetus, seeing it as kind of a foreign intruder.

• Ask your doctor for a simple blood test for your blood type and Rh status if you’ve ever been pregnant or may become pregnant in the future.

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