Objective Studies on the risk of preterm birth among women exposed to heavy lifting during pregnancy have provided conflicting results, and data on high-level exposures are limited. We addressed this limitation in a prospective cohort study on preterm birth among women with more extreme levels of ergonomic hazards at work, especially lifting.

Methods The study included 378 employed pregnant women referred to an occupational hospital clinic between 1984 and 2010, because of ergonomic hazards at work. Secondary exposure in terms of heavy lifting and/or lifting of unwieldy loads (n = 201) was defined by a medical doctor based upon a detailed description of current work. Preterm birth was defined as gestational age <37 weeks at delivery. The control group included 343,106 pregnancies of employed pregnant women in the general population, with permanent address in Denmark, pregnant during the same period of time. Birth information was extracted from the Danish Medical Birth Register, and occupational information from Statistics Denmark. Risk estimates computed by both logistic regression (OR, 95% CI) and Cox regression (HR, 95% CI) were adjusted for the mother's age at delivery, nationality, parity, smoking habits, socioeconomic status and the year of conception.

Results The estimates for ergonomic hazards indicated an increased risk of preterm delivery (OR: 1.21, CI: 95%: 0.86; 1.71 and HR: 1.20, 95% CI: 0.87; 1.66), but the analyses were not significant. In a sensitivity analysis specifically for lifting (n = 201), however, borderline significant associations were found (OR: 1.49, 95% CI: 0.96; 2.30 and HR: 1.45, 95% CI: 0.96; 2.18).

Conclusion Heavy lifting at work during pregnancy may confer an increased risk of preterm birth. Exposures encountered in our population may have been more strenuous than encountered in earlier large population based studies. There may be indications for advising pregnant women against heavy lifting at work during pregnancy.