RESUMO
We present a case of a dichorionic/diamniotic twin pregnancy in which one twin presented with ultrasound findings suggestive of molar changes in the placenta. The placenta of twin A seemed to be grossly enlarged and cystic, and twin A was small for gestation. After an inevitable abortion, a detailed histological and genetic evaluation was performed on the fetus and placenta from twin A, including traditional cytogenetic techniques, microsatellite marker analysis, fluorescent in situ hybridization, and p57 immunostaining. It was determined that twin A was a chimera with a biparental XX cell line and an androgenetic XY cell line. The 2 cell lines were present in both the placenta and the fetus. The patient later developed and was treated for persistent gestational trophoblastic disease, which has been shown to have an increased risk after an androgenetic conception. Cases of mosaicism or chimerism involving an androgenetic cell line may be difficult to diagnose histologically but are critical to identify because of the increased risk for persistent gestational trophoblastic disease. Therefore, we emphasize the importance of using multiple molecular, cytogenetic, and immunohistochemical techniques when diagnosing cases involving such unusual placental abnormalities. To our knowledge, this is the first reported case of persistent gestational disease after a fetal chimera.