. "Women of blood type O are more prone than women of types A and B to making IgG anti-A and anti-B antibodies, and these IgG antibodies are able to cross the placenta.[citation needed] For unknown reasons, the incidence of maternal antibodies against type A and B antigens of the IgG type that could potentially cause hemolytic disease of the newborn is greater than the observed incidence of ABO dise" . . .