The major influence on birth weight is gestational age. At any given week of gestation, however, size varies enormously. There is no specific cut-off that separates abnormally large or small babies for gestational age from normal. Instead, function alters as a continuum across the weight distribution. Small babies are prone to hypoxia, acidosis, and stillbirth. Large babies on the other hand are associated with prolonged labor and mechanical problems. The optimum size for fetal survival is 1 to 1.5 standard deviations above the mean, whereas cesarean section rates are lowest when the fetal weight is 0.5 to 1.5 standard deviations below the mean. Antenatal detection of both very small and very large babies is difficult and imprecise. Expectant management is therefore preferable unless there are very specific indications for emergency or operative delivery.