Rh Incompatibility

Because not everyone has the substance know as the Rh factor, it becomes an issue during pregnancy. While it may seem counterintuitive, the problem arises when the mother is Rh-negative and her fetus is Rh-positive, and the problem can affect a future fetus as well. A simple injection can guard against life-threatening conditions in a fetus.

Approximately 90 percent of people are born with a substance on their blood cells known as the Rh factor, so called because a similar substance is found on the blood cells of rhesus monkeys. Individuals who have this factor on their blood cells are called Rh-positive; those without it are called Rh-negative. The presence of the Rh factor is inherited from one's parents.

Problems can occur in the fetus if the mother is Rh-negative and the fetus is Rh-positive. Normally, some fetal blood escapes from the placenta and enters the mother's blood. If both the mother and fetus are Rh-positive, this leakage of fetal blood causes no problems. However, since this fetal blood containing the Rh factor is a foreign substance to a mother who is Rh-negative, the mother's body forms antibodies against the Rh factor.

The body normally forms antibodies in response to foreign substances -- usually bacteria and viruses that may be harmful. These antibodies act by destroying the foreign substance, thus protecting the body against their harmful effects. A person who forms antibodies against a substance is called sensitized to that particular substance.

If the mother forms these antibodies against Rh-positive blood, the antibodies cross from the mother into the fetus and start destroying fetal blood cells. This leads to a serious condition in the fetus called erythroblastosis, which may lead to anemia, heart failure, and even stillbirth.

When an Rh-negative mother becomes sensitized, it is usually at the time of the birth of her first Rh-positive baby. This baby is generally unaffected. But the mother keeps her antibodies against Rh-positive blood for life. If she has another pregnancy with an Rh-positive fetus, this fetus may be affected.

During early pregnancy, the doctor performs a blood test to determine if the mother has Rh-positive or Rh-negative blood. If the mother is Rh-negative, the father is also tested to determine his Rh factor. If he is Rh-positive, the baby may be Rh-positive as well.

As a precaution, to prevent the mother from becoming sensitized to Rh-positive blood, her doctor may recommend she receive an injection of a substance called human anti-Rh immune globulin at 28 weeks of pregnancy. If her baby is found at birth to be Rh-positive, the mother will receive another injection within two days of delivery.

The human anti-Rh immune globulin prevents the mother's body from developing Rh antibodies that would cause complications. An injection may also be given if the mother has an amniocentesis, a miscarriage, or an ectopic pregnancy. If it is found that the mother has already developed Rh antibodies, the pregnancy is closely monitored to determine if blood transfusions are necessary.

The Rh factor is an issue only during pregnancy. On the next page you'll learn about diseases that are either exclusive to pregnancy or more pronounced during this time.