Ninety five placentae from normal term and 'normal' preterm pregnancies have been analysed by morphometry. The growth of the whole placenta, of the microscopic placental constituents and of the villous surface area has been measured in placentae from uncomplicated pregnancies from 10 weeks of gestation to term to provide data for comparison with values from placentae from pathological pregnancies. No evidence for placental senescence is found in the placentae from normal pregnancies up to term although the rate of growth of the villous surface area decreases gradually from approximately 34 weeks gestation. Evidence is given to suggest that in normal pregnancy villi continue to elaborate until term. The relationship of villous surface area to fetal growth is assessed.