First Trimester: Blood Typing and RBC Antibody Screen
Blood typing is usually done during the first trimester or the first prenatal visit. It is used to determine a pregnant woman's blood group, to establish whether she is A, B, AB, or O, and whether she is Rh-positive or Rh-negative. A pregnant woman should know her blood type. [See Blood Typing for more information on how this is performed.]
Blood typing is especially important during pregnancy because a mother and her fetus could be incompatible. For example, if the mother is Rh-negative but the father is Rh-positive, the fetus may inherit the Rh antigen from the father and be Rh positive. If the blood types of mother and baby are different, as in this example, the mother may produce antibodies (antiglobulins) that react with antigens (proteins or factors) on the fetus' red blood cells. The antibodies may cross the placenta and cause destruction of the baby's red blood cells, resulting in a serious condition referred to as hemolytic disease of the newborn (HDN). Although the first Rh-positive baby is unlikely to become ill, the antibodies produced during that first pregnancy may affect subsequent Rh-positive babies.
To greatly reduce the likelihood that an Rh-negative mother will develop this antibody, she may be treated with an injection of Rh immune globulin at approximately 28 weeks' of pregnancy. The Rh immune globulin binds to and "masks" the fetus's Rh antigen and prevents the mother from developing antibodies against the Rh antigen. 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 developed 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 done during the first trimester and repeated during the third trimester (between 28 and 29 weeks of pregnancy) determines if potentially harmful antibodies are 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 RBCs have antigens that the mother's antibody could target. If so, then the fetus' RBCs may also have antigens that may be targeted. In this case, the healthcare practitioner will likely monitor the mother's antibody level and the health of 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.