In preeclampsia, the placenta grows slowly along its minor axis

A small placental surface area at birth is associated with an increased risk of hypertension in the offspring in later life. Preeclampsia is associated with impaired implantation and with increased blood pressure in the offspring. We hypothesized that preeclampsia would be associated with a small placental surface area. We studied placental size in 6410 deliveries at the Helsinki University Central Hospital during 1934-44. 284 of the pregnancies were complicated by preeclampsia. 1855 were complicated by hypertension without proteinuria. The area of the placental surface was estimated from two diameters that were routinely recorded, a maximal diameter and a lesser one at right angles to it. Compared to normotensive pregnancies, the placentas from pregnancies complicated by preeclampsia had a reduced surface area and the surface was more oval. The thickness, however, was increased. When the two diameters were analyzed together, preeclampsia was not associated with the length of the maximal diameter, but was strongly associated with a short lesser diameter (p<0.0001). This was a graded relation: the shorter the lesser diameter, the greater the risk for, and severity of, preeclampsia. Placentas from pregnancies complicated by hypertension without proteinuria had a reduced surface area, with short lesser and maximal diameters. Processes that underlie preeclampsia may be closely related to the amount of placental tissue on the minor axis of the placenta. We postulated that placental growth is polarized from the time of implantation, so that growth along the major and minor axes is qualitatively different.