Rh factor can complicate pregnancy

A generation ago, the term "Rh factor" struck fear among mothers-to-be. Prospective mothers knew that if this factor in their blood was opposite that of the fetus (if mother's was negative and baby's was positive), that the baby was terribly at risk. But since 1968, a blood product given as a shot has relieved the risk in most cases.

You may have heard the terms "Rh positive" or "Rh negative." They refer to a kind of protein present in some blood cells. Most people have the protein, meaning they are Rh positive. A simple blood test can tell you whether you are positive or negative.

Actually, whether you are positive or negative does not matter when it comes to having a baby, except when mother and baby carry different Rh factors.

How the Rh factor can cause problems

A few women with Rh negative blood can have problems if they're carrying an Rh positive baby. The mother's body may react as if the baby's blood is a foreign substance and a threat. The mother's body may fight against this perceived threat by making antibodies that attack the fetal blood. This is called hemolytic disease and it can result in serious illness, brain damage or death to the fetus or the newborn.

Complications can be prevented

Complications can be prevented with a shot of Rh immunoglobulin (Rhogam), usually given around the 28th week of pregnancy and then again within 72 hours of delivery. This drug is often given in the muscle of an arm or buttock and is perfectly safe for a pregnant woman.

Rhogam is a blood product that keeps an Rh negative mother from making the antibodies that attack the fetal blood. This drug will not work in those rare cases in which the mother's body is already producing Rh antibodies.

The treatment is good only one pregnancy at a time. In other words, if you had a shot like this for your first pregnancy, you may still need to have another one for additional pregnancies if you're Rh negative and carrying an Rh positive baby. Rhogam is also important to receive within 72 hours of a miscarriage or pregnancy loss.

Rh factor can affect more than one pregnancy

Once a woman's body has formed these antibodies, they don't go away. Many times an Rh negative woman with an Rh positive baby will develop the antibodies after the baby is born. A very few of these women will have problems during the last three months of pregnancy.

Rh negative women need to receive the Rhogam treatment any time there is a miscarriage, induced abortion, or ectopic pregnancy because these are situations in which the woman's blood can start making these antibodies. A woman can also be sensitized (begin making the antibodies) during a blood transfusion.

Once an Rh negative woman develops the antibodies, the RhIg treatment won't help. If treatment with RhIg isn't possible, the pregnancy will be closely monitored. Sometimes blood transfusions are given to the fetus while in the uterus or afterbirth. Sometimes early delivery is recommended.

How do you know if you're already sensitized?

A blood test called an antibody screen will show if an Rh negative woman has developed antibodies to Rh positive blood.

Date last reviewed: October 2002.

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