An Rh test can determine whether you have Rh-positive or Rh-negative blood. Read on to learn why testing for Rh factor is important during pregnancy.

By Nicole Harris
Rostislav_Sedlacek/Shutterstock

The Rh factor test is routine for nearly all pregnant women. It’s usually conducted when your blood sample is taken during your first prenatal care visit. The results will determine if your blood has “Rh factor”—a protein on the surface of red blood cells. 

While a majority of Americans have Rh factor (making them “Rh positive”), 15 percent of the white population and 7 percent of the African-American are Rh-negative. Find out why the Rh factor test is important, and how the results of the Rh test could impact your pregnancy. 

Why Do You Need an Rh Test in Pregnancy? 

So why do women need the Rh test during pregnancy? Doctors must check whether the Rh factor, a protein on the surface of red blood cells, is present in your blood. 

Being Rh positive won’t affect the pregnancy. But if you’re Rh negative and the baby’s father is Rh positive, the baby could inherit the Rh-positive blood. Because of this Rh incompatibility, the mother’s body will consider the baby's Rh factor a foreign object, and it may react by producing antibodies that attack the baby's blood cells. These antibodies can cause complications ranging from newborn jaundice to stillbirth.

Early detection of Rh factor problems can dramatically reduce such dangers, so it’s vital that mothers-to-be get tested as soon as possible. 

How To Test for Rh Type

Blood is taken through a small needle, usually from a vein in the arm. The sample is then sent to a lab for analysis.Test results are available within a few days. You probably won’t need to take any measures for an Rh-positive test (Rh factor is present).

If your blood sample reveals that the Rh factor is not present, you are considered Rh-negative. Your blood will be taken again at 28 weeks to be tested for antibodies. If you don't have antibodies, you'll be given an injection of Rh-immunoglobulin (RhIg), which prevents your body from producing them and thus protects the baby from harm. If you do have Rh antibodies, you will be monitored closely. If antibody levels become too high, special measures, including blood transfusions to the baby and early delivery, may be taken.

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