Placental mesenchymal dysplasia and an estimation of the population incidence.

Placental mesenchymal dysplasia (PMD) is a rare placental malformation of as yet undetermined etiology. We report a single center's experience of this diagnosis and present an estimation of the population incidence. Within our institution, all placentae are examined within a pathology department that provides a dedicated perinatal service. In this study, we evaluated the incidence of PMD over a period of 18 years following the description and recognition of PMD as a pathological diagnosis. During the period 1991-2009, only two cases were identified amongst over 95 000 deliveries at our institution. This series of placental examinations is by far the largest in a normal population within which the occurrence of PMD is reported, and the resulting incidence of only 0.02 per 1000 deliveries is some 10 times less than that which has previously been estimated.