BACKGROUND: Laboratory workers are commonly exposed to chemical, biological and physical agents. They also may adopt poor postures for long periods and be engaged in moving and handling. These factors may increase the risk of adverse pregnancy outcome in female laboratory workers. AIMS: To assess whether laboratory work during pregnancy increases the risk of adverse pregnancy outcomes. METHODS: The 1990-2006 Finnish Medical Birth Registry was used to identify all singleton newborns of all Finnish laboratory workers (n = 5425) and those of teachers (n = 21 438) as the reference population. The main outcomes were sexual differentiation (female gender), low birth weight, high birth weight, preterm delivery, post-term delivery, small-for-gestational age (SGA), large-for-gestational age and perinatal death. The generalized estimating equation (GEE) analysis was used to estimate odds ratios (ORs) adjusted for maternal age, parity, marital status and maternal smoking during pregnancy. RESULTS: In the GEE analysis, the risk of low birth weight (adjusted OR: 1.27, 95% CI: 1.08-1.45) and SGA (adjusted OR: 1.27, 95% CI: 1.02-1.52) was higher in laboratory workers than in teachers. Correspondingly the prevalence of high birth weight (>/=4000 g) was lower in newborns of laboratory workers (adjusted OR: 0.90, 95% CI: 0.83-0.98). The prevalence of post-term deliveries was close to being significantly higher among newborns of laboratory workers (adjusted OR: 1.16, 95% CI: 1.00-1.31). CONCLUSIONS: This large population-based study provides evidence that laboratory work may be associated with reduced foetal growth.