Going through UW's questions I found one that asks about ABO antibodies and alloimmunization during pregnancy (mother O Rh+ father AB Rh+ first pregnancy). Their answer says that antibodies to ABO antigens do not cross the placenta as they are IgM.

Goljan's Step 1 notes have the opposite explanation. They clearly say that anti AB IgG antibodies exist in some people without prior exposure and do cross the placenta and cause ABO hemolytic disease of the newborn.

I'm taking Goljan's word over UW's. Anybody know more?