The most common proble with Rh incompatibility may arise during pregnancy. Normally, there is no direct contact between maternal and fetal blood when a women is pregnant.
However, if a small amount of Rh+ blood leaks from the fetus through the placenta into the blood stream of an Rh- mother, the mother will start to make anti-Rh agglutinates.
Since the greatest possibility of fetal blood transfer occurs at delivery, a newborn baby will be unaffected. If the mother becomes pregnant again, however, her anti-Rh agglutinate can cross the placenta and make their way into the blood stream of the fetus.
If the fetus is Rh-, there is no problem, since Rh- blood does not have the Rh agglutinate. If the fetus is Rh+, hemolysis may occur in the fetal blood.
The hemolysis brought on by fetal-maternal incompatibility is called hemolytic disease of the newborn (HDN), also called erythroblastosis fetalis.
When a baby is born with this condition, blood is slowly removed and replaced a little at a time, with Rh- blood. It is even possible to transfuse blood into the unborn child if the problem is diagnosed before birth.
Not too long ago, Hemolytic disease of the newborn was a major cause of fetal and newborn deaths, but now its occurrence is very rate.
Hemolytic disease of the newborn is prevented by giving all Rh- mothers an injection of an anti-Rh gamma globulin preparation (RhoGAM) soon after delivery, miscarriage, or abortion.
These agglutinins bind to the fetal agglutinates, if they are present, so the mother's immune system cannot respond to the foreign agglutinogens by producing agglutinates (antibodies).
Thus the fetus of the next pregnancy is protected. Since Rh+ mothers do not make anti-Rh agglutinins, their babies are not at risk.
ABO incompatibility between mother and fetus rarely causes problems because most of the ABO aggluutinins are too large to cross the placenta.
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