Erythroblastosis, fetal

Rh incompatibility may occur when an Rh negative mother carries an Rh-positive fetus. At the time of delivery, a small amount of the baby s Rh-positive blood may gain access to the maternal circulation. In response, the immune system of the mother produces anti-Rh antibodies. During the subsequent pregnancy, the fetus is exposed to these antibodies as they cross the placenta. If this fetus is also Rh-positive, then the anti-Rh antibodies attack the fetal erythrocytes and cause hemolytic disease of the newborn (erythroblastosis fetalis). This may occur in about 3% of second Rh-positive babies and about 10% of third Rh-positive babies. The incidence continues to increase with subsequent pregnancies. 

The rhesus D antigen occurs in 84% of all white individuals, who are therefore Rh-pos-itive. If an Rh-positive child is born to an Rh-negative mother, fetal erythrocytes can enter the mother s circulation during birth and lead to the formation of antibodies (IgG) against the D antigen. This initially has no acute effects on the mother or child. Complications only arise when there is a second pregnancy with an Rh-positive child, as maternal anti-D antibodies cross the placenta to the fetus even before birth and can trigger destruction of the child s Rh-positive erythrocytes [fetal erythroblastosis). 

---