To assess the effects of trace metals on birth weight and gestational age among newborn babies of mothers without occupational exposure. The subjects examined were 142 newborn babies (71 males and 71 females) delivered at two university hospitals in Shanghai, China and their parents. Relationships of newborn birth weight and gestational age to concentrations of arsenic, lead, cadmium, manganese, zinc, and cobalt in maternal and cord blood were investigated. Birth weight was 3379.5 +/- 440.8 (2090-4465) g and the gestational age was 39.7 +/- 1.3 (35-43) weeks. Stepwise regression analysis indicated that, in the male newborn, birth weight and gestational age were inversely related to the logarithm arsenic concentration (4.13 +/- 3.21 microg/l) in mothers' whole blood. Arsenic might have a negative influence on newborn birth weight and gestational age at a relatively low exposure level. This effect was observed in male but not female babies, suggesting a sex differential in susceptibility to arsenic at an early stage of development. Although birth weight is believed to be related to gestational age, arsenic may directly affect both birth weight and gestational age.