Learn about Rh compatibility in you and your baby's blood, and why you need to identify any possible incompatibilities early.

By Dr. Laura Riley
October 18, 2009

You will have a blood test early in your pregnancy to determine whether your blood has a protein factor called Rh. If it does, you are Rh-positive, which is true for about 85 percent of the U.S. population. If it does not, you are Rh-negative. If you are Rh-negative, it alerts the practitioner to a potential incompatibility between your blood and your baby's blood.

Having Rh-negative blood doesn't pose a problem during pregnancy unless the baby's father is Rh-positive. If that's the case, then there's a chance that your baby could be Rh-positive and that your blood and your baby's are incompatible. (If both you and your baby are Rh-negative, then all is well.)

During pregnancy, red blood cells from your baby can cross the placenta into your bloodstream. This isn't usually a problem in a first pregnancy, but if this is your second pregnancy with an Rh-positive baby, there's a chance that your red blood cells will recognize this different blood type and rise up in revolt, creating an immune response against your own baby because your Rh-negative blood won't tolerate Rh-positive blood cells. (This can also happen after a miscarriage or an abortion with an Rh-positive fetus.) Your practitioner will give you an injection of Rhogam, an Rh-immune globulin, to halt your body's production of antibodies that might attack your baby's blood. This is typically done in the 28th week of your pregnancy, with a follow-up dose within 72 hours after birth if in fact your baby's blood tests show that he's Rh-positive. You may also receive a Rhogam shot if you're Rh-negative and experience any bleeding or if you have a medical procedure during which blood could be transferred, like amniocentesis.

If you have this immune response and don't receive the shot of Rhogam, your blood cells may treat the Rh-positive cells of your baby like a virus or any other intruder and make antibodies against them. The antibodies can cross the placenta to reach your baby and destroy his circulating red blood cells, causing severe anemia in your unborn child. In a small percentage of cases this can result in your baby's death.

Originally published in You & Your Baby: Pregnancy.

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