Placental mesenchymal dysplasia (PMD) is a rare placental vascular anomaly which resembles partial molar pregnancy by 2-D ultrasonography. It is challenging but clinically important to distinguish between them in order to avoid unnecessary termination of pregnancy. A patient was referred to our centre at 13 weeks of gestation and 2-D ultrasound of the placenta showed a widespread vesicular pattern mixed with normal appearing placenta. Amniotic fluid volume was normal, and the fetus appeared to be an appropriate size for gestation without obvious structural abnormalities. 3-D reconstruction imaging of the placenta showed a large multi-cystic area arising from the chorionic plate which was adjacent to normal-appearing placenta. 3-D imaging rendered with ‘inversion mode’ revealed multiple fluid-filled structures with different sizes and appearances. Her serum hCG level was slightly elevated. All findings taken together, we suspected PMD rather than partial molar pregnancy. Histological examinations of the placenta after termination at 15 weeks confirmed the diagnosis.