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(Accepted 13 March 1987)

Abstract In a study of 2000 unselected pregnant women the development ofa mature placental appearance (grade 3) on ultrasonography by 34-36 weeks' gestation, observed in 15% of cases, was associated with maternal smoking, low parity, low maternal age, and being white. These women had an increased risk of problems during labour and their babies had an increased risk oflow birth weight, poor condition at birth, and perinatal death. The women were Peterborough Maternity Unit, Peterborough PE3 6BP JEAN PROUD, SCM, MTD, sister in charge of scanning department National Perinatal Epidemiology Unit, Radcliffe Infirmary, Oxford OX2 6HE ADRIAN M GRANT, DM, MRCOG, epidemiologist Correspondence to: Dr Grant.