What Is the Rh Factor?
Just as there are different major blood groups, such as A and B type blood, there also is an Rh factor. The Rh factor is the type of protein on the red blood cells. Most people have the Rh factor — they are Rh positive. Others do not have the Rh factor — they are Rh negative.

When Does the Rh Factor Cause Problems?
The Rh factor causes problems when an Rh-negative person's blood comes in contact with Rh-positive blood.

An Rh-negative woman can become sensitized if she is pregnant with an Rh-positive fetus. If a pregnant woman's blood group is Rh negative, knowing whether the father is Rh positive or Rh negative will help find the risk of Rh sensitization.

During pregnancy, mother and fetus do not share blood systems. Blood from the fetus can cross the placenta into the mother's system, though. When this occurs, a small number of pregnant women with Rh-negative blood who carry an Rh-positive fetus will react as if it were allergic to the fetal blood. Then, they become sensitized by making antibodies. These antibodies go back to the fetus and attack the fetal blood. They break down the red blood cells and produce anemia (lack of iron in the blood). This condition is called hemolytic disease.

In a second pregnancy with an Rh-positive fetus, the antibodies are more likely to cause hemolytic disease in the fetus.

Sensitization can occur any time the fetus' blood mixes with the mother's blood.

How Can Problems Be Prevented?
A simple blood test can tell a woman's blood type and Rh factor. Another blood test, called an antibody screen, can show if an Rh-negative woman has developed antibodies to Rh-positive blood.

Rh immunoglobulin (RhIg) is a blood product that can prevent sensitization of an Rh-negative mother.

RhIg, first used in 1968, is safe and easily obtained. Its use can prevent sensitization in almost all cases.

When Is RhIg Used?
During Pregnancy and After Delivery
If a woman with Rh-negative blood has not been sensitized, her doctor may suggest that she receive RhIg around the 28th week of pregnancy to prevent sensitization for the rest of the pregnancy.

Shortly after birth, if the child has Rh-positive blood, the mother should be given another dose of RhIg. In almost all cases, this treatment prevents the woman from making antibodies to the Rh-positive cells she may have received from her fetus before and during delivery. No treatment is needed if the father or baby is also Rh negative.

Other Reasons RhIg May Be Given
Amniocentesis. Amniocentesis is a procedure in which a small amount of amniotic fluid (the fluid in the sac that surrounds the fetus) is withdrawn from the mother's uterus through a needle for testing. If and when this is done, fetal Rh-positive red blood cells could mix with a mother's Rh-negative blood. This would cause her to produce antibodies. Thus, RhIg is given.

Postpartum Sterilization. An Rh-negative mother may receive RhIg after a birth even if she decides to have her fallopian tubes tied and cut to prevent future pregnancies.

What Happens If Antibodies Develop?
Once a woman develops antibodies, RhIg treatment does not help. Doctors are finding ways to save infants who get hemolytic disease. A mother who is Rh-sensitized will be checked during her pregnancy to see if the fetus is developing the disease.

Finally . . .
To protect against Rh sensitization, all pregnant women should have a blood test done at an early stage of pregnancy.

This excerpt from ACOG's Patient Education Pamphlet is provided for your information. It is not medical advice and should not be relied upon as a substitute for visiting your doctor. If you need medical care, have any questions, or wish to receive the full text of this Patient Education Pamphlet, please contact your obstetrician-gynecologist.