Relative and attributable risks were calculated to assess the potency of different maternal factors associated with large-for-gestational age (LGA) babies in individuals and in the population as a whole. In multiparous women the most important factor was the fast fetal growth rate demonstrated in previous pregnancies. Non-smoking also made a large contribution to the LGA population (43.65%) and the risk to individual patients was doubled. The relative risk for height steadily rose with increasing stature, but the effect was much more marked for weight. Heavy women made a considerable contribution to the LGA population (26%) and the effect of increasing weight is such that very heavy women (more than +2 SD) are nine times more likely to have an LGA baby than those of average weight. The relative risk also rose sharply with increasing parity. Nevertheless, 27.5% women in our LGA group were having their first baby. The possibility that a very obese non-smoking primiparous woman may be bearing a very large baby should not be overlooked.