Although many women work during pregnancy, the effect of maternal job experience on pregnancy outcome is controversial. We investigated whether work-related physical exertion increases a woman's risk of delivering a preterm, low birthweight infant. We studied 773 employed, pregnant women included in the National Longitudinal Survey of Labor Market Experience, Youth Cohort (NLSY), a nationally representative sample of young adults. Data concerning work status, job title during pregnancy, and other factors affecting the outcome of pregnancy were obtained from the NLSY. Assessment of physical exertion was based on job title, using an established catalogue of occupational characteristics. Women in jobs characterised by high physical exertion experienced a higher rate of preterm, low birthweight delivery, defined as maternal report of delivery more than 3 weeks early and birthweight under 2,500 g (adjusted RR = 5.1, 95% CI = 1.5, 17.7). These findings support a policy of limiting work-related physical exertion during pregnancy.