I'm pregnant. What do I need to know about Rh factor or Rh incompatibility?

Answers from Roger W. Harms, M.D.

Rhesus (Rh) factor is an inherited trait that refers to a specific protein on the surface of red blood cells. If your blood has the protein, you're Rh positive — the most common Rh factor. If your blood lacks the protein, you're Rh negative. Your health care provider can determine your Rh factor with a basic blood test. Your Rh factor doesn't affect your health. However, your pregnancy needs special care if you're Rh negative and your partner is Rh positive.

During pregnancy — or, more likely, during delivery — your blood and the baby's blood may mix. If you're Rh negative and your baby is Rh positive, your body may produce Rh antibodies to fight the baby's blood. The antibodies are generally harmless during the first pregnancy. However, if you have a subsequent pregnancy with an Rh positive baby, your Rh antibodies may attack the baby's red blood cells. The subsequent Rh positive baby may develop Rh disease, a life-threatening condition that could require a blood transfusion through the umbilical cord during pregnancy or immediately after delivery.

If you're Rh negative, your health care provider will likely give you an Rh immune globulin injection at about 28 weeks into your first pregnancy. You'll need another Rh immune globulin injection shortly after delivery if the baby is Rh positive. The Rh immune globulin blocks your body's recognition of Rh positive cells, which prevents any problems with Rh incompatibility. An Rh immune globulin injection may be needed in other cases as well, such as after an episode of vaginal bleeding, an amniocentesis, an ectopic pregnancy or a miscarriage.

If you're Rh negative, you'll likely need an Rh immune globulin injection during any subsequent pregnancies and after the delivery of each Rh positive baby as well.

Mother's Rh factorFather's Rh factorBaby's Rh factorPrecautions
Rh positive Rh positive Rh positive None
Rh negative Rh negative Rh negative None
Rh positive Rh negative Could be Rh positive or Rh negative None
Rh negative Rh positive Could be Rh positive or Rh negative Rh immune globulin injections

If you're Rh negative, remind your health care team of your Rh status when you seek medical care during pregnancy.

May. 30, 2009 See more Expert Answers