The size of the placenta at birth has been shown to predict blood pressure levels and hypertension in later life. In some studies small placental size predicts raised blood pressure. In other studies raised blood pressure is predicted by high placental weight in relation to birthweight. This latter association is thought to result from compensatory placental expansion in mid gestation. It is well known that if a pregnant ewe is undernourished the fetal lamb is able to extend the surface area of its placenta to extract more nutrients from the mother. This response, however, can only occur if the ewe was well-nourished at the time of conception. Compensatory placental enlargement results in a larger lamb at birth than is seen in pregnancies where the ewe was well-nourished throughout. Inducing placental enlargement in mid-gestation is standard practice in sheep farming.

Almost all published studies in humans are based on placental weight at birth. We have examined the effects of placental surface area in the Helsinki Birth Cohort Study. In the past it was recognised that the surface of the placenta is oval rather than round. In the Helsinki study all placentas were weighed at birth and two diameters of the surface were recorded, a maximal one and a lesser one at right angles to it. Using these diameters it is possible to estimate the area and shape of the placental surface and its thickness. We find that the surface area predicts hypertension more strongly than the weight. Both a small and a large placental area predict the disorder. The association with a large placental area is only found in the offspring of tall mothers, which is consistent with the observations in sheep.

We are requesting permission to photograph the surface of all stored placentae, so that the surface area can be estimated. We recognise that storage will have shrunk the placentae, but one of us is a placentologist and is familiar with the issues. In the proposed study the predictors will be placental weight, the maximal and lesser diameter, measured surface area and measured thickness, body size at birth, length of gestation and mother's body size. The outcome will be systolic and diastolic blood pressure. The analyses will take into account child's age, sex and body size at the time of blood pressure measurement, together with the family's socio-economic status.

We estimate that it will be possible to photograph 300 placentas a week. The Director has told us there are 9,000 placentae and we should therefore be able to complete the photography in 30 weeks.