Objective A significant inverse relationship between blood pressure and birth weight is firmly established. This association may be the result of fetal adaptations to an adverse intrauterine environment Further markers of intrauterine growth include the weight of the placenta and the placental ratio (the ratio of placental weight to birth weight). A number of studies suggest that a decreased placental weight or an elevated placental ratio may be independent risk factors for subsequent high blood pressure. The overall evidence for this is, however, inconclusive, The purpose of the present study was to clearly define the relationships between placental weight, placental ratio and subsequent blood pressure during childhood.Design Prospective cohort study of 2507 singleton children, born at term during 1989-1992, Blood pressures were recorded at ages 1, 3 and 6 years, using a semiautomated oscillometric device.Results Inverse relationships existed between both systolic and diastolic blood pressure and placental weight, adjusted for current weight at ages 1, 3 and 6 years. The relationships between placental weight and systolic blood pressure were statistically significant at ages 1 and 3 years. There was no consistent relationship between placental weight and later blood pressure within birth weight categories. No clinically or statistically significant association was seen between the placental ratio and either systolic or diastolic blood pressures at any age.Conclusions Birth weight, rather than placental weight or their ratio, is the early life factor most importantly related to subsequent blood pressure in childhood. (C) 2001 Lippincott Williams & Wilkins.