AIMS: To examine pregnancy outcomes in women doing laboratory work. METHODS: Using data from the Danish National Birth Cohort (1997-2003), the authors conducted a prospective cohort study of 1025 female laboratory technicians and 8037 female teachers (as reference). The laboratory technicians were asked about laboratory work tasks during pregnancy in an interview (at around 16 weeks of gestation). Pregnancy outcomes were obtained by linking the cohort to the national registers. Hazard ratios (HRs) of late fetal loss and diagnosing of congenital malformations were calculated by using Cox regression, and odds ratios (ORs) of preterm birth and small for gestational age were calculated by using logistic regression. RESULTS: Overall, there were no significant differences in pregnancy outcomes between laboratory technicians and teachers. However, we found that laboratory technicians working with radioimmunoassay or radiolabelling had an increased risk of preterm birth (OR = 2.2, 95% CI 0.8 to 6.2 for radioimmunoassay, and OR = 1.9, 95% CI 0.8 to 4.6 for radiolabelling) and "major" malformations (HR = 2.1, 95% CI 1.0 to 4.7 for radioimmunoassay, and HR = 1.8, 95% CI 0.9 to 3.7 for radiolabelling). The ORs of preterm birth doubled for women working with these tasks every day or several times a week. When an exposure matrix was applied, an increased risk of "major" malformations for exposure to organic solvents was seen. CONCLUSIONS: The results did not indicate any high risk of reproductive failures in laboratory technicians in general. Exposure to radioisotopes may carry a high risk of preterm birth and congenital malformations. This finding deserves further investigation.