In cases of Rh incompatibility, a baby's red blood cells have a substance called the Rh D factor, and the mother's blood cells do not. In medical terms, the baby is Rh positive and you are Rh negative. If some of the baby's red blood cells leak into your system, your body may produce antibodies to the Rh D factor. These antibodies can affect your unborn baby or the next Rh-positive baby you have.