Blood group complications can always arise when mother and fetus have different blood groups.
The most common complications of this type are associated with the Rhesus blood group. Where the mother is Rh-negative (i.e. does not have the Rh antigen) but the fetus is Rh-positive, the mother will produce antibodies against the fetal red blood cells if fetal blood enters her bloodstream either during pregnancy or at birth. These antibodies are then carried across the placenta to the fetus, where they burst the fetal red blood cells (illustration). As a result, the fetus becomes anemic and in rare cases may die while still in the mother’s abdomen.