Six new cases of placental mesenchymal dysplasia are presented and the findings compared to those reported in 16 similar cases published in the literature. Mesenchymal dysplasia was suspected when a placental scan showed a partial mole with a fetus of normal size and normal karyotype. Three fetuses of this series and nine out of 18 cases from the literature review also presented with Beckwith-Wiedemann syndrome features. This placental anomaly is more commonly associated with a 46,XX karyotype. Comparable placental histopathological features in cases of mesenchymal dysplasia with or without congenital anomalies diagnostic of Beckwith-Wiedemann syndrome suggest that in some of these cases the overgrowth of specific fetal tissue is limited to the placenta and the fetus remains unaffected. Histological similarity between mesenchymal dysplasia and cellular chorioangioma suggests a common embryologic origin for both these placental abnormalities. Ultrasound/Doppler serial investigations indicate that the circulatory imbalance is due to hypovascularization of the dysplastic lobules, found in mesenchymal dysplasia. This induces the progressive aneurysmal and varicose dilatation of chorionic vessels, however, these anatomical transformations are not associated with a change in resistance to flow in both uterine and umbilical circulations nor with an excess of obstetrical complication when the fetus is anatomically normal.