Calorie supplementation during pregnancy is associated with heavier babies in a rural Guatemalan sample. This paper outlines other possible maternal and environmental factors related to birthsize and suggests that increased birthweight is the prime factor in reducing infant mortality among babies born to supplemented mothers. Caloric supplementation during pregnancy has less effect on birthweight than do maternal height and weight at conception, indicating that efforts to improve maternal nutrition should begin in childhood for optimum infant survival, and that during pregnancy, small thin mothers should be supplemented to prevent the birth of 'small for date' babies.