Blood types are either A, B, AB, or O, and Rhesus (Rh) positive or negative. Both the mother and baby may experience problems if their blood types are different, or if the mother has antibodies that will react with factors on the baby’s 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 reacts with the Rh-positive factor on her baby’s blood. The mother’s immune system does not usually come into contact with the baby’s blood until labour, so it is unusual for her first Rh-positive baby to become ill. However the antibodies produced during that first pregnancy may cause problems in a future Rh-positive pregnancy.
To greatly reduce the likelihood that an Rh-negative mother will develop this antibody, she may be given an injection of ‘anti-D gamma globulin’ at approximately 28 and 34weeks gestation, as well as shortly after the birth of an Rh-positive baby. Additional injections may be necessary during the pregnancy if there is any reason to suspect that her blood may have come into contact with that of the Rh-positive baby (e.g. amniocentesis, chorionic villous sampling or an abdominal injury). The injection is of an antibody which mops up any antigen present from the baby’s blood cells, so preventing the mother’s immune system from reacting to it.
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.
Limitation of antibody screening
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. If the mother has received an injection of ‘anti-D gamma globulin’ within the previous six months, the antibody screen may produce a positive result.