If you just found out you're pregnant, one of the first — and most important — tests you should expect is a blood-type test. This basic test determines your blood type and Rh factor. Your Rh factor may play a role in your baby's health, so it's important to know this information early in your pregnancy.
About the Rh Factor
People with different blood types have proteins specific to that blood type on the surfaces of their red blood cells (RBCs). There are four blood types — A, B, AB, and O.
Each of the four blood types is further classified based on the presence of another protein on the surface of RBCs that indicates the Rh factor. If you carry this protein, you are Rh positive. If you don't carry the protein, you are Rh negative.
Most people — about 85% — are Rh positive. But if a woman who is Rh negative and a man who is Rh positive conceive a baby, there is the potential for a baby to have a health problem. The baby growing inside the Rh-negative mother may have Rh-positive blood, inherited from the father. Approximately half of the children born to an Rh-negative mother and Rh-positive father will be Rh positive.
Rh incompatibility usually isn't a problem if it's the mother's first pregnancy because, unless there's some sort of abnormality, the fetus's blood does not normally enter the mother's circulatory system during the course of the pregnancy.
However, during delivery, the mother's and baby's blood can intermingle. If this happens, the mother's body recognizes the Rh protein as a foreign substance and might begin making antibodies (protein molecules in the immune system that recognize, and later work to destroy, foreign substances) against the Rh proteins.
Other ways Rh-negative pregnant women can be exposed to the Rh protein that might cause antibody production include blood transfusions with Rh-positive blood, miscarriage, and ectopic pregnancy.
Rh antibodies are harmless until the mother's second or later pregnancies. If she is ever carrying another Rh-positive child, her Rh antibodies will recognize the Rh proteins on the surface of the baby's blood cells as foreign, and pass into the baby's bloodstream and attack those cells. This can lead to swelling and rupture of the baby's RBCs. A baby's blood count can get dangerously low when this condition, known as hemolytic or Rh disease of the newborn, happens.
Preventing and Treating Rh Disease of the Newborn
In generations past, Rh incompatibility was a very serious problem. But now, significant medical advances can help prevent complications from Rh incompatibility and treat any newborn affected by Rh disease.
Today, when a woman with the potential to develop Rh incompatibility is pregnant, doctors give her a series of two Rh immune-globulin shots during her first pregnancy. The first shot is given around the 28th week of pregnancy and the second within 72 hours after giving birth. Rh immune-globulin acts like a vaccine, preventing the mother's body from producing any potentially dangerous Rh antibodies that can cause serious complications in the newborn or complicate any future pregnancies.
A dose of Rh immune-globulin also might be given if a woman has a miscarriage, an amniocentesis, or any bleeding during pregnancy.
If a doctor finds that a woman has already developed Rh antibodies, her pregnancy will be closely monitored to make sure that those levels are not too high.
In rare cases, if the incompatibility is severe and the baby is in danger, a series of special blood transfusions called exchange transfusions can be done either before the baby is born (intrauterine fetal transfusions) or after delivery. Exchange transfusions replace the baby's blood with blood with Rh-negative blood cells. This stabilizes the baby's level of red blood cells and minimizes further damage caused by Rh antibodies already circulating in the baby's bloodstream.
Because of the success rate of the Rh immune-globulin shots, exchange transfusions are needed in fewer than 1% of Rh-incompatible pregnancies in the United States today.
If Rh Disease Is Not Prevented
Rh incompatibility rarely causes complications in a first pregnancy and does not affect the health of the mother. But if Rh antibodies develop, they could be dangerous to a fetus during later pregnancies. Rh disease can cause severe anemia, jaundice, brain damage, and heart failure in a newborn. In extreme cases, it can cause the death of the fetus because too many RBCs have been destroyed.
If you're not sure what your Rh factor is and think you're pregnant, it's important to start regular prenatal care as soon as possible — including blood-type testing. With early detection and treatment of Rh incompatibility, you can focus on more important things — like welcoming a new, healthy baby.
Reviewed by: Elana Pearl Ben-Joseph, MD
Date reviewed: October 2014