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Rh Factor

Why Your Doctor Needs to Know If You Have an Rh-negative Blood Factor

By Sharon Waldrop

A+, A-, B+, B-. You probably haven't seen more plus and minus signs since high school or your last series of home pregnancy tests.

What is the "+" or "-" sign after a blood type of A, B, O or AB? It is indicative of an Rh factor and it is critical that an expectant mother and her health care provider are aware of an Rh-negative blood factor. It's not critical for the mother's sake, but for the sake of her unborn child or future unborn children. The prenatal blood screen performed during the first prenatal visit will determine Rh status, with results available within one to two business days and no risk to the mother or fetus.

A blood type with a positive Rh factor will produce a protein that inhibits the surface of red blood cells. Rh-negative red blood cells lack this protein. The Rh factors were named after they were first discovered in the blood of the rhesus monkey.

If an Rh-negative woman and an Rh-positive man conceive a child together, the fetus' Rh factor will be positive or negative, with no harm to an Rh-negative baby and most likely no harm to a Rh-positive baby. However, if the baby's Rh factor is positive, during delivery the fetus' Rh-positive blood cells may enter the mother's bloodstream. In very rare situations, this can happen during pregnancy. Since an Rh-negative mother's bloodstream does not contain the Rh factor, her body will produce antibodies to fight off the foreign red blood cells, thus threatening the red blood cells of future unborn children. If an Rh-negative mother conceives an Rh-negative baby, there are no foreign red blood cells present and no threat of future fetal blood cell destruction.

An injection to protect the fetuses of Rh-negative mothers was developed in 1968. An injection of Rh immunoglobulin (Rhlg) given to the mother during the 28th week of pregnancy can usually prevent harm to a developing fetus. This injection is commonly known as a Rhogam injection. If the baby is born with an Rh-positive factor, the mother will receive a second injection within 72 hours after birth to protect the next baby. If the new baby is Rh-negative, no second injection is necessary. The injection is given via the mother's hip and it might sting a bit. An injection is also given to Rh-negative mothers who miscarry.