Dizygotic twin pregnancy with a normal fetus and a nodular embryo associated with a partial hydatidiform mole.

Although twin pregnancies complicated by a coexisting complete hydatidiform mole are uncommon, those with partial hydatidiform mole (PHM) are exceedingly rare; there are only several well-documented cases diagnosed antenatally. Here we present the first case of a twin placenta containing a nodular embryo associated with PHM diagnosed on routine placental examination. This dizygotic twin pregnancy featured viable embryos at 8 weeks' gestation, death of 1 embryo at 12 weeks, and delivery of a healthy infant by caesarean section at 28 weeks because of worsening maternal reflux nephropathy. Macroscopic and microscopic placental examination and fluorescence in situ hybridization showed one part of the placenta to be diploid and the other to contain a vanishing triploid embryo and a PHM, which had eluded antenatal ultrasound diagnosis. Careful pathologic examination of vanishing twins and their placentas may disclose an unexpected PHM, which can be associated, albeit infrequently, with persistent gestational trophoblastic disease or a trophoblastic tumor.