Rh Sensitization

A woman with Rh-negative blood who becomes pregnant by an Rh-positive man creates the possibility of having a Rh-positive fetus inside a Rh-negative mother. In rare circumstances, if the blood of the fetus mixes with the mother’s blood (they’re not supposed to!), the mother makes “antibodies” against the fetus’s blood. These antibodies, if produced early enough in the pregnancy and in sufficient quantities, attack the fetus’s RBCs and destroy them. Prior to 1968, Rh sensitization was the primary cause of Hemolytic Disease of the Newborn (HDN). In the worst cases of HDN, the fetus becomes severely anemic. The anemia may produce severe edema (swelling), a condition called “hydrops”, and possibly even death. Many of these “blue babies” (from lack of oxygen) were born prior to 1968. It is fortunately, a very rare condition now.

Darrow, one of the early researchers on this problem, was a Rh-negative mother whose Rh-positive children were affected. Thanks to her and several other researchers a passive immunization was produced and licensed in 1968 which has all but eradicated the problem.

If you are Rh-negative, your midwife or doctor may suggest that you get a shot of Rh(o) immune globulin (aka RhoGam or RhIG) at about 28 weeks of pregnancy. If your baby is Rh-positive, you should receive a shot after the birth. The shot contains antibodies against the Rh protein. If the fetus’s blood does enter the mother’s blood stream, the fetal cells are destroyed by the antibodies from the Rh shot before her own body recognizes the fetal blood. This prevents the mother from producing her own antibodies against Rh-positive blood.

Without the RhIG shot, if fetal cells enter the mother’s bloodstream in sufficient numbers, the mother will produce antibodies against Rh-positive blood. She will have become “sensitized”. Once sensitized, always sensitized. If the sensitized mother has another Rh-positive baby, and during that pregnancy fetal blood cells enter her bloodstream, she may activate the production of Rh antibodies. If this happens, these antibodies can cross the placenta and attack the fetus’s blood cells.

If you are Rh-negative and you are absolutely-positively-100%-certain that the father of your baby is Rh-negative, then the baby is also Rh-negative. In this case, you will not need to receive the shot. If the father of your baby is Rh-positive, there is a greater than 50% chance that your baby has Rh-positive blood. Since it is obviously very difficult to test the fetus’s blood before birth, most midwives and doctors recommend that all Rh-negative women receive the RhIG shot at about 28 weeks. Also, if you ever become pregnant with an Rh-positive father, you may need RhIG in future pregnancies.

RhIG should also be given if you have a miscarriage or other type of significant hemorrhage during your pregnancy. It is also usually given if you have an amniocentesis. This prevents “sensitization” by the Rh-positive protein, which could prevent Rh problems in future pregnancies. Sensitization usually does not occur in these circumstances due to the low volume of blood which is transfered from the baby to the mother……but it can.

Is it absolutely necessary to get the RhIG shot before the baby is born? No. Since almost all sensitizations occur at birth (when the baby’s blood is more likely to enter the mother’s blood stream), if a Rh-negative women receives the shot after birth, chances are that she will be adequately protected. But a small percentage of women will have significant fetal-maternal blood transfers during pregnancy, and begin to produce antibodies against their fetus.

If you decide that you do not want the RhIG shot during pregnancy, you should have your blood drawn periodically to look for the presence of Rh antibodies. You should have an antibody screen at the beginning of pregnancy and again at 28, 32, 36, 38 and 40 weeks. If you begin to develop Rh antibodies (that is, you have become sensitized), your baby may be at risk for hemolytic disease. Even if you don’t get the RhIG shot during pregnancy, if you give birth to a Rh-positive baby, you should have the shot one or two days after birth to protect the babies of future pregnancies.

One last word on the topic, most first-time pregnant women who are Rh-negative do not need to worry about this problem. Only after exposure to Rh-positive blood (the process of “sensitization”) can a Rh-negative women produce antibodies against her Rh-positive baby’s blood. This first sensitization usually occurs at the time of the delivery of a woman’s first baby. At this time large amounts of the baby’s blood can enter the mother’s bloodstream. The mother then produces antibodies against the Rh protein, which she may produce during future pregnancies.

What is the chance that you are Rh-positive? It is probable that the gene which controls this blood protein mutated decades or centuries ago in the Basque population of Spain. Due to “cross-pollination” (and some new mutations) over the years the gene has spread beyond this group. The percentage of Rh-negative blood in the Asian population is about 1%. In Africans and Caucasians it is about 15%. Among the Basques, it’s over 30%.


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