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Q&A: Is it possible for an Rh Negative blood type to switch to Rh Positive?

Question: In my last two pregnancies, my blood test results showed that I have Rh Negative blood, and I received the customary antibody injections during pregnancy and after the delivery of my babies. I’m pregnant again and I have a new doctor. One of my blood tests came back stating that I am O+ (Blood Type O, Rh Positive) and that I’m not Rh Negative. How can my blood type change? What dangers are involved if I receive the antibody while pregnant if I am not Rh Negative? I was already given an injection in the first trimester because of bleeding. Is it possible for an Rh Negative blood type to switch by itself to Rh Positive?

Answer: “Rh,” another marker for types of blood besides A, B, AB, and O, can be either Rh Negative or Rh Positive. If a patient with Rh Negative blood were to receive blood from a Rh Positive donor, she would make antibodies to the Rh Positive blood given to her. The same thing happens if she has a baby who, through inheritance from the father, would have Rh Positive blood, like the father. At the time of delivery when the placenta separates, there is some mixing of maternal and fetal blood. Some fetal blood does get into the maternal bloodstream, and this is just like receiving a blood transfusion from a Rh Positive donor. The mother would then make antibodies to this Rh Positive blood. But since the mother is Rh Negative, it’s no big deal, because these antibodies will only attack Rh Positive blood, of which the mother has none.

Until the next pregnancy.

If the next baby she’s carrying is once again Rh Positive (thanks to Dad), then her old anti-Rh Positive antibodies, small enough to pass from her circulation through the filtering of the placenta into the baby’s bloodstream, will attack the baby’s red blood cells. This causes severe anemia in the baby, called Erythroblastosis and can be lethal. Up to 15 percent of women have Rh Negative blood, but now we can give Rh Immune-globulin, big bulky anti-Rh Positive antibodies which fool the body into thinking the defense has already been launched. Therefore, no antibodies are made by the mother. And since these are bulky molecules, they won’t pass on to the fetus, like the mother-made ones do. These “blocking” antibodies therefore protect the unborn baby.

Now that we have that out of the way, I can address your question. No, it isn’t possible to change blood types. But Rh Positive women are identified by their Rh Positive red blood cells. There is a very weak variant called the “Du” variant. It is actually an Rh Positive antigen, which would make a woman “Rh Positive,” but since it is very weakly expressed in tests, it’s possible to be missed and mistakenly read as Rh Negative. Today, any Rh Negative woman should be tested further for the Du variant. If it’s positive, then the blood type is corrected to Rh Positive, and no Rh Immune-globulin need be given. If this is the case for you, then you received these shots for nothing with your previous pregnancies. Fear not, getting Rh immune-globulin when you’re Rh Positive does no harm, except for your pocketbook. But this is a recent test, so it probably was unavailable during your previous pregnancies. So it may be that your new doctor has it right and that your previous doctors didn’t have the technology back then to check for the Du variant.

Of course, it doesn’t hurt to ask your health care provider just in case your blood tubes got mixed up with someone else’s!

Although you didn’t ask, I’ll also tell you that if you were truly Rh Negative, you would need the Rh immune-globulin even after a miscarriage, since there can be mixing of maternal and fetal blood there too. We also give this shot to Rh Negative mothers who have undiagnosed bleeding during their pregnancy, car accidents, and version (converting a breech to a head-first baby).

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