Partial molar transformation of the placenta of presumably monozygotic twins

A pregnancy with one normal female fetus and a placenta that was divided in to halves, one normal the other molar, is described. Genetic analysis shows the molar component to be hyperdiploid/tetraploid but having an identical DNA composition as the normal twin. Because there was no trophoblastic prol iferation and the hyperdiploid cells were confined to the villous stroma, a nd because the molar component was still being perfused by diploid vessels from the normal twin, we believe the mole is derived from polyploidization of the mesenchymal epiblast in a monozygotic twin pregnancy.