In birth certificate data for Massachusetts resident births from 1978 to 1982, 12–27% of births purportedly under 31 weeks of gestation were probably misclassified, i.e. had birthweight ≥2500g. Correcting for maldistribution of births removed 34% and 23%, respectively, of black and white births with reported gestational ages <36 weeks but with implausible weights. Percentages of unknown and incomplete reports of last menstrual period were also significantly higher for blacks. After adjustment, preterm black infants weighed less than whites at each gestational age. The proportion of infants <2500g born at term (≥37 weeks gestation) was higher (although not significantly) among blacks. These findings are consistent with hypotheses that low socioeconomic status negatively affects the rate of intrauterine growth.