Rh blood types were discovered in 1940. The Rh system was named after rhesus monkeys since they were initially used in the research. The greatest risk of injury with the Rh group is as between a mother and her developing fetus. Mother-fetus incompatibility occurs when a mother is Rh negative and the father is Rh positive. Maternal antibodies can cross the placenta and destroy fetal red blood cells. The risk increases with each pregnancy. With preventative treatment, the risk of injury can be greatly reduced. However, Rh blood type incompatibility is still the leading cause of potentially fatal blood-related problems of newborns. In the United States, 1 out of 1,000 babies are born with this condition. The first time an Rh negative woman becomes pregnant, there usually are not incompatibility difficulties for her Rh positive fetus. However, the second and subsequent births are likely to have life-threatening problems.