The Rh factor is an inherited protein found on the surface of red blood cells. Most people have this protein and are called Rh-positive. Some people, however, don’t have protein; they’re called Rh-negative.
In the U. S., about 15% of the white population, 5-8% of the African-American and Hispanic populations, and 1-2% of the Asian and Native American populations are Rh-negative. Being Rh-negative doesn’t affect a person’s health in any way.
Problems can arise, however, when an Rh-negative mother and an Rh-positive father conceive an Rh-positive baby. When this occurs, some of the fetus’s Rh-positive red blood cells may get into the mother’s bloodstream during pregnancy, labor and birth. Because red blood cells containing the Rh factor are foreign to the mother’s system, her body tries to fight them off by producing antibodies against them. This is called sensitization.
Once a woman becomes sensitized, her Rh antibodies can cross the placenta and destroy some of the red blood cells of an Rh-positive fetus. In a first pregnancy with an Rh-positive baby, there usually are no serious problems because the baby often is born before the mother is sensitized, or at least before the mother produces many Rh antibodies. However, a sensitized woman continues to produce Rh antibodies throughout her life. This means that in a second or later pregnancy, an Rh-positive baby is at risk for more severe Rh disease. Fortunately, treatment usually can prevent Rh disease.
A simple blood test can tell if a woman is Rh-negative. Every woman should be tested at her first prenatal visit, or before pregnancy, to find out if she is Rh-negative. Another blood test can show if an Rh-negative woman has become sensitized.
An unsensitized Rh-negative pregnant woman can be treated with shots of a purified blood product called Rh immune globulin (RhIg) to prevent sensitization. She most likely receives RhIg at 28 weeks of pregnancy and again within 72 hours of giving birth if a blood test shows that her baby is Rh-positive. She does not need an injection after delivery if her baby is Rh-negative. Some health care providers recommend an additional RhIg injection if a woman’s pregnancy goes past her due date.
An Rh-negative woman should be treated with RhIg after any situation in which the fetal red blood cells can mix with her blood, including: miscarriage, ectopic pregnancy, induced abortion, amniocentesis, chorionic villus sampling (CVS), abdominal trauma, external cephalic version (when the health care provider attempts to turn a breech-position baby into head-down position before labor).
An Rh-negative woman does not need treatment with RhIg if blood tests show that the baby’s father is Rh-negative. If the father is Rh-negative, the baby is Rh-negative. An Rh-negative baby is not at risk of Rh disease.