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
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
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
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
last revision spring 2010