Data of 10,514 singleton births collected over a ten year period at a single hospital are analysed and models linking birthweight and gestational age with mortality and morbidity defined by low Apgar scores are constructed and compared. Based on these models, charts of mortality and morbidity are drawn and compared with common charts of birthweight centiles. Classification rules for newborns at risk are defined by iso-mortality contours, marginal birthweight centiles, and birthweight centiles adjusted by gestational age, respectively, and compared using receiver operating characteristic (ROC) curves. The results suggest that, as far as neonatal mortality is concerned, classification rules based on simple marginal birthweight centiles perform almost as well as iso-mortality contours and considerably better than birthweight centiles adjusted for gestational age.