People with the same blood group have no particular difficulty conceiving healthy children together. According to About.com, the major risk involving blood groups and pregnancy comes when an Rh-negative mother conceives a child with an Rh-positive father. In this case, it is possible the mother's immune system will attack the fetus as if it is an invader.
Human blood factors are grouped as A, B, AB and O, which are important to know for the purposes of transfusions and transplants. In addition to these factors, the blood is also described as Rh-positive or negative. As explained by About.com, the Rh factor has no effect on human health except during pregnancy.
Around 15 percent of the general population is Rh-negative. When an Rh-negative mother conceives a child with an Rh-positive father, the mother's immune system sometimes becomes sensitized and begins to produce antibodies that cross the placenta and damage the fetus' red blood cells. This hemolytic disease, as About.com describes, can cause anemia, brain damage and death. This danger makes it all the more important to seek adequate medical care during pregnancy and, since the danger increases with subsequent pregnancies, always provide a full and accurate medical history to the doctor.