Nuffield Institute for Medical Research, University of Oxford, U.K. There are good reasons why the main features of fetal growth are of concern to farmers and to physicians. The appearance of runts in a litter reduces productivity, and small-for-dates infants cause problems in obstetric management in about 4 % of human pregnancies. It is not easy to make quantitative measurements of intrauterine growth retardation. At present we are still limited in our ability to make accurate longitudinal studies, i.e. sequential measurements on the same fetus in utero. Direct measurements have been made in animals by application of radioopaque markers or by temporary removal of the fetus for measurement. More recently ultrasound measurements of the diameter of the fetal head, or of the total fetal mass estimated from transverse B-scan sections at right angles to the longitudinal axis have offered an acceptable means of estimating fetal growth (Campbell, 1974). Yet it still