Rh status, either positive or negative, is determined through a blood typing test. If your red blood cells carry a specific protein, you are Rh positive. If this protein is lacking, you are Rh negative. This status normally does not affect health. One exception is if an Rh negative woman conceives a baby with an Rh positive man. The baby has a 50% chance of having Rh positive blood. When an Rh negative mother carries an Rh positive baby, this is called Rh incompatibility. If a few fetal cells get into the Rh negative mother’s blood, she will form antibiodies against the Rh protein. This will affect any future Rh positive babies she may have causing destruction of their blood cells. When this incompatibility is discovered during pregnancy, it can easily be treated (the mother is given injections to prevent the development of antibodies to the baby’s Rh positive blood).

A situation where the mother's blood type is rH negative, and the father's is rH positive. This is not a concern during a first pregnancy, but if the mother is sensitized to rH-positive blood during the pregnancy, subsequent pregnancies may attack by her immune system. A shot is given to the rH negative mother at the end of any pregnancy, regardless of length or outcome, to prevent the production of harmful antibodies.

A condition in which a baby's blood type is incompatible with the mother's. If carefully monitored and treated, women whose Rh factor is incompatible with their fetus's will usually give birth to a healthy baby.