Mother and child have their own separate circulation. Both blood streams meet intimately in the placenta, separated by one thin cell layer. Here small leaks occur, especially later in pregnancy, so that maternal and fetal blood is mixed to a certain degree. The pregnant woman may produce antibodies against certain antigens in the fetal blood and those antigens pass the placental barrier and enter the fetal circulation.

It is very rare that the mother makes antibodies against the regular blood groups A and B. But antibodies against the Rhesus groups (C, D and E) are more common, especially D. This means that if a Rhesus negative woman (d) is pregnant of a Rhesus positive child (D) she may produce anti-D antibodies which enter the fetal circulation and may destruct the fetal blood cells. If the woman has not been exposed to Rhesus positive blood before (blood transfusion with Rhesus positive blood or previous pregnancy) the effect on the fetus is negligible because the antibody titre does not reach a sufficient level to harm.

The leakage of fetal blood into the maternal circulation is provoked by delivery when uterine contractions may disrupt the physical barrier between both blood streams, and during termination when mechanical stimulation does the same. For this reason the woman should be given an injection of anti-D-immunoglobulin to catch eventual antibodies in her circulation so that she does not produce her own antigens.

The injection is not necessary in the following cases:

1. Young pregnancy (under 7 weeks). This is the only certain condition.

2. The fetus is also Rhesus negative (this is the case if the father is Rhesus negative).

3. The woman did already produce antigens, for instance after blood transfusion.

4. The woman will certainly have no further pregnancies.

Rhesus Anti-D-immunoglobulin is available in 2 ml ampules containing l000 units. For a pregnancy from 8 to 12 weeks 375 units is enough, for further advanced pregnancies l000 units should be given.

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last revision spring 2010