What Is the Rh Factor?

By Bailey Miller
Reviewed by Alyssa Quimby, M.D.
May 22, 2024

The Rh factor is a protein found on the outer layer of red blood cells. Some people have it, and some people don’t.

You should be given a blood test for Rh factor as part of routine prenatal testing in the first trimester. If you have the Rh factor on your blood cells, you’re Rh positive, and if you don’t have the Rh factor, you’re Rh negative.

If you’re Rh negative but your fetus is Rh positive, that’s called Rh incompatibility. In some pregnancies, this incompatibility can cause your body to attack the fetus’s blood cells with anti-Rh antibodies if their blood comes in contact with yours. This can cause health problems for a fetus or newborn, including anemia, jaundice, and sometimes death.

The good news is that there are ways to prevent these problems, especially if you test early. For instance, there’s a medication pregnant people can take to prevent the formation of antibodies and ultimately keep you and your baby safe. If you’re Rh negative, your healthcare provider will talk to you about when you will receive this medication—most of the time at the beginning of the third trimester.