Blood types are either A, B, AB, or O, and Rh positive or negative. Both the mother and baby may experience problems if their blood types are different, or if the mother has antibodies (antiglobulins) that will react with antigens (proteins or factors) on the baby’s red blood cells.

The best known example is when an Rh-negative woman is pregnant with an Rh-positive baby. The woman’s immune system can develop an antibody that attaches to the Rh-positive antigens on her baby’s red blood cells and target them for destruction. Although it is unusual for her first Rh-positive baby to become ill, the antibodies produced during that first pregnancy will affect any future Rh-positive babies. To greatly reduce the likelihood that an Rh-negative mother will develop this antibody, she may be given a routine injection of Rh immune globulin (RhoGam) at approximately 28 weeks gestation. Additional injections may be necessary during the pregnancy if she has an amniocentesis, chorionic villi sampling, or an abdominal injury, and after delivery if the baby is Rh-positive. Before each injection is given, an antibody screen (indirect antiglobulin test) is performed to make sure that the woman has not already created Rh antibodies.

In addition to Rh-negative women who have had an Rh-positive baby, any woman who has had a blood transfusion or had prior pregnancies may produce an antibody to blood factors other than Rh that can potentially harm an unborn baby. An antibody screen during the first trimester is ordered to determine if potentially harmful antibodies are already present in the mother’s blood. If a harmful antibody is detected, the baby’s father should be tested, if possible, to see if his blood has antigens that react with the mother’s antibody. If it does react, then the baby’s may also. If there is a possibility that the antibody could react with the baby’s, then it is advisable for the health care provider to monitor the mother’s antibody level and the progress of the baby for the duration of the pregnancy. Signs that the fetus is becoming ill may necessitate that it receive treatment before birth (such as an intrauterine transfusion) or require an early delivery.

Limitations of the antibody screen:
An antibody may be present but in an amount too low to be detected.
The baby’s blood may react with the antibody even if the test is negative.

Related Pages
Tests: Indirect antiglobulin test