The majority of second-trimester partial moles are found in association with triploidy. Rarely are they associated with tetraploidy or other aneuploidies and, to our knowledge, this is the first reported case of the prenatal diagnosis of partial mole in a pregnancy presenting with trisomy. The patient was referred at 21 weeks of gestation after a routine ultrasound examination had shown fetal and placental features suggesting a partial mole triploidy. Owing to the severe structural malformations and poor prognosis, the parents requested termination. Prenatal and postnatal cytogenetic investigations demonstrated an additional chromosome 13. Histopathological examination of the placenta showed focal areas of villous edema but no evidence of trophoblastic dysplasia. The maternal serum human chorionic gonadotropin level was within the normal range at all times. This case shows that trisomy can resemble a triploid partial mole in utero without the potential long-term risk to the mother of persisting trophoblastic disease, as villous molar changes can obviously develop without trophoblastic dysplasia.