SDN Members don't see this ad. (About Ads) So Rh factor fetal-mother incompatibility makes sense. Why doesn't this happen with ABO? The mother can be OO and the baby be AO or BO. The mother would have anti-A and anti-B. Why no erythroblastosis fetalis? Wikipedia says something like most anti-A and anti-B are IgM which can't cross the placenta. Again, why are they igM when anti-Rh are IgG?