Rh (Rhesus) factor is a blood protein that plays a critical role in some pregnancies. People without Rh factor are known as Rh negative, while people with the Rh factor are Rh positive. If a woman who is Rh negative is pregnant with a fetus who is Rh positive, her body may make antibodies against the fetus's blood. This can cause Rh disease in the baby, also known as hemolytic disease of the newborn, or erythroblastosis fetalis. In severe cases, Rh disease leads to brain damage and even death. Since 1968 a vaccine has existed to prevent the mother's body from making antibodies against the fetus's blood.
Rh factor is an antigen found on the red blood cells of most people. Rh factor, like the blood types A, B, and O, is inherited from one's parents. A simple blood test can determine blood type, including the presence of the
Rh factor. About 85% of white Americans and 95% of African-Americans are Rh positive. A person's own health is not affected by the presence or absence of Rh factor.
Rh factor is important only during a pregnancy in which an Rh negative woman is carrying a fetus who might be Rh positive. This can occur when an Rh negative woman conceives a baby with an Rh positive man. The gene for Rh positive blood is dominant over the gene for Rh negative blood, so their baby will be Rh positive. If the Rh positive father also carries the gene for Rh negative blood, his babies have a 50% chance of inheriting Rh negative blood and a 50% chance of inheriting Rh positive blood. If both parents are Rh negative, their babies will always be Rh negative. In order to protect their future babies from Rh disease, all women of childbearing age should know their Rh status before becoming pregnant.
Rh factor in pregnancy
The danger of Rh disease begins when the mother's Rh negative blood is exposed to the baby's Rh positive blood. This mixing of blood can occur at the time of birth, and after an abortion or miscarriage. It may also happen during prenatal tests such as amniocentesis and chorionic villus sampling. More rarely, blood from the mother and fetus may mingle during pregnancy, before birth. When this contact occurs, the mother's body responds by building antibodies to fight the foreign Rh blood protein. The mother's blood is now said to be "sensitized" against Rh factor blood.
Once a mother's blood has become sensitized, her antibodies will attack the blood of any Rh positive fetus that she carries. The antibodies will destroy the fetus's red blood cells. If this happens, the infant will suffer from Rh factor incompatibility. It will become anemic, a condition caused by a reduction in red blood cells and marked by weakness and fatigue. Severe anemia can lead to heart failure and death. The breakdown of red blood cells will also cause the formation of a reddish-yellow substance known as bilirubin. An infant with high levels of bilirubin will look yellowish. This is known as jaundice.
Rh disease is usually not a problem during a first pregnancy. This is because the Rh negative mother probably will not become sensitized until her blood mixes with the baby's blood during birth. Her baby will be born before her blood can produce antibodies against the baby's Rh positive blood. Once a mother is sensitized, however, any future babies with Rh positive blood will be at risk for Rh disease.
PREVENTION AND TREATMENT. Since 1968 a vaccine has existed to prevent sensitization from occurring. This is the best way to eliminate Rh disease. Available as an injection, the vaccine is called Rh immune globulin (brand name RhoGAM). It blocks the action of the antibodies and prevents the mother's blood from attacking the baby's blood. To be effective, the vaccine must be given any time fetal blood mixes with maternal blood: after birth, abortion, miscarriage, or prenatal tests like amniocentesis and chorionic villus sampling. The vaccine is typically given within 72 hours of any of these events. Since mixing of the blood may also occur during the last three months of pregnancy, the vaccine is also administered at 28 weeks of pregnancy.
A pregnant woman who has already been sensitized from a previous pregnancy will want her doctor to carefully monitor the level of antibodies in her blood throughout her pregnancy. As long as the antibody levels remain relatively low, no problem exists. But if those levels rise, the fetus will need special attention. High antibody levels mean that the fetus's red blood cells are being attacked and destroyed.
A fetus whose red blood cells are being destroyed will need a blood transfusion while it is still in the uterus. Two or three transfusions may be necessary before the baby is born. If the fetus shows signs of illness close to its anticipated birth, the physician may elect to deliver the baby early, either by inducing birth or by cesarean section. The baby will then receive a transfusion after birth.
ELIMINATING RH DISEASE. Until the introduction of the Rh immune globulin vaccine, Rh disease could not be prevented. About 45 babies per 10,000 births developed the disease each year before widespread use of the vaccine in the early 1970s. Use of the Rh immune globulin vaccine has reduced the chances of the mother becoming sensitized from approximately 12-13% to 1-2%.
Nevertheless, the disease is not completely eradicated. Further steps must be taken, since this is a preventable disease. The majority of cases of Rh disease are the result of women not receiving the vaccine at the appropriate time. Poor women without health insurance, who are likely to lack adequate prenatal care, are especially vulnerable to this oversight. Older women may have become sensitized before the vaccine was available; foreign-born women may not have had access to the vaccine.
Amniocentesis—A procedure in which a sample of the fluid in the amniotic sac is removed with a needle for genetic testing.
Antigen—A substance, usually a protein or carbohydrate, that stimulates a response from the immune system.
Bilirubin—Reddish-yellow substance formed by the breakdown of red blood cells.
Chorionic villus sampling—A procedure in which a sample of the membrane surrounding the fetus is taken for genetic testing.
Prenatal test—Procedure done to determine the presence of disease or defect in a fetus.
Sensitization—Occurs when a mother's blood produces antibodies against the blood of her Rh positive fetus.
Salem, Leon. "Rh incompatibility." eMedicine Journal 2 (May 16, 2001): <http://www.emedicine.com/emerg/topic507.htm>.
"Rh Factor and Pregnancy." University of Michigan Health System Website. 1999. July 9, 2001. <http://www.smartmoms.org/prenatal-tests/test8.html>.