Relative and attributable risks were calculated to assess the potency of different maternal factors associated with intrauterine growth retardation in individual pregnancies and in the population as a whole. In multiparous women the most important factor for the total population was the slow fetal growth rate demonstrated in previous pregnancies. Moderate or severe preeclampsia gave a higher individual risk, but because of its low prevalence the contribution to small-for-gestational-age (SGA) pregnancies as a whole was low. Smoking and hypertension without preeclampsia each trebled the individual risk, and made a substantial contribution to the overall population of SGA babies. Other less potent factors were low maternal height, weight, and weight gain in pregnancy. When adjustment had been made for maternal size, smoking, pathological factors, and siblings' birthweight, the effect of social class became insignificant. The remaining "causes' of SGA pregnancies are likely to be single factors of large effect such as major congenital abnormalities and uterine abnormalities. If the pathological factors of smoking, hypertension and preeclampsia could be prevented, the number of SGA pregnancies in this population would be reduced by about 50%.