Rhesus disease (or Rh disease) is caused when Rh-positive red blood cells from a fetus enter the maternal circulation of an Rh-negative woman. This usually happens at the time of delivery, but it can occur at other times during pregnancy, such as spontaneous miscarriage or abortion. These cells are recognized as foreign to the mother's immune system, and antibodies are formed to destroy them. In the next pregnancy, these antibodies can cross the placenta and cause anemia (low blood count) in the developing fetus. Rhesus-immune globulin is administered routinely at seven months in pregnancy and after delivery in Rh-negative women. Although 99 percent effective in preventing Rh disease, reports from the 1990s indicate that 1 to 6 infants per 1,000 live births have evidence of the effects of Rh disease. Pregnant women with an antibody concentration of greater than 1:16 are monitored with serial ultra-sounds and amniocenteses to measure possible destruction of fetal blood. In some cases, the fetus can be given red blood cells while still in the womb; this is done through an intrauterine transfusion.
"Prevention of Rh D Alloimmunization." American College of Obsetricians and Gynecologists Practice Bulletin 4 (1999).
Ventura, Stephanie, Joyce Martin, Sally Curtin, and T. J. Mathews."Births: Final Data for 1997." National Vital Statistics Reports 47 (1999):1-96.