Third Trimester: Antibody Screen
Blood types are either A, B, AB, or O, and Rh positive or negative. A pregnant woman should know her blood type. [See Blood Typing for more information on how this is performed.] This is because both the mother and her baby may experience problems if their blood types are different from each other, or if the mother has antibodies (antiglobulins) that react with antigens (proteins or factors) on the fetus' red blood cells. This may result in a serious condition referred to as Hemolytic Disease of the Newborn (HDN).
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 fetus' red blood cells and target them for destruction. Although the first Rh-positive baby is unlikely to become ill, the antibodies produced during that first pregnancy will affect 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 villus sampling, or an abdominal injury, and after delivery if the baby is Rh-positive. Before each injection is given, an antibody screen 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 determines 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 fetus' may also. If the antibody could react with the fetus', the health care provider should monitor the mother's antibody level and the fetus for the duration of the pregnancy. Signs that the fetus is becoming ill may necessitate treatment before birth (such as an intrauterine transfusion) or an early delivery.
Although Rh incompatibility has more severe consequences, one of the most common causes of HDN is actually an incompatibility between the mother's and baby's ABO blood groups, not the Rh factor. However, the RBC antibody screen cannot be used to predict whether HDN will occur because antibodies to the ABO blood groups are naturally occurring.