Cystic placentomegaly on a second-trimester ultrasound.

Placental mesenchymal dysplasia (PMD) is a rare disorder of the placenta characterized by placentomegaly, cystic vesicles, and dilated chorionic blood vessels. Clinically and pathologically, it closely resembles partial molar pregnancy and complete hydatidiform mole with a coexistent healthy fetus, both of which are associated with malignant trophoblastic disease. PMD, however, has no risk of malignant trophoblastic disease and can result in the birth of a normal fetus, highlighting the need for clinician awareness of PMD in order to avoid unnecessary termination of a viable and potentially healthy fetus.