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 factor is critical for the sake of an unborn child. The prenatal blood screen performed during the first prenatal visit will determine Rh status.

If an Rh negative woman and an Rh positive man conceive a child, 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 of Rh immunoglobulin (Rhlg) given to the mother during the 28th week of pregnancy can usually prevent harm to a developing fetus. 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.