The estriol/creatinine ratio was determined in a first morning urine sample in 1808 pregnancies in the 32nd wk of gestation. The ratio obtained correlated closely with birthweight and showed a linear increase with percentile birthweight group. There was a 25-fold increase in perinatal mortality in those pregnancies with a ratio below the 10th percentile; all pregnancies ending in stillbirths with birthweight below the 10th percentile gave such a ratio. The single estrogen/creatinine ratio allowed early detection of a group of pregnancies at high risk of intrauterine growth retardation or major total abnormality without inconvenience for the patient.