Rh incompatibility occurs when the mother's blood type is Rh negative and her fetus' blood type is Rh positive.
If some of the fetus' blood passes into the mother's blood stream, her body will produce antibodies in response.
These antibodies could pass back through the placenta and harm the fetus' red blood cells, causing mild to serious anemia in the fetus.
Reviewed By: Neil K. Kaneshiro, MD, MHA, Clinical Assistant Professor of Pediatrics, University of Washington School of Medicine, Seattle, WA. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.