The relations between some hereditary and environmental factors and the variation in infant birth weight were estimated by multiple linear regression analyses on a sample of 276 Scandinavian single term pregnancies. Gestational age explained 10.9%, fetal factors (maternal and paternal birth weights and fetal sex) 7.9%, maternal factors (parity, pre-pregnancy weight and height) 5.5% and external factors (adjusted weight gain and smoking) 5.8% of the variation in infant birth weight. It is concluded that maternal and paternal birth weights are rather poor predictors of infant birth weight. Together they explain only 5.6% of the variation in infant birth weight at term.