Standing at work and preterm delivery

OBJECTIVE: Guidelines in some European countries and the United States suggest that pregnant women should avoid prolonged standing and heavy lifting in the workplace during the second and third trimester of pregnancy. However, results from epidemiological studies on this topic are ambiguous. The aim of this study was to evaluate the influence of standing and walking at work in the second trimester on preterm delivery in a population with a low frequency of other workplace hazards. SUBJECTS AND DESIGN: A prospective cohort of 8711 women with singleton pregnancies was established during 1989 through 1991. Information was collected during the 16th week of pregnancy about medical and obstetrical history, general lifestyle factors and exposures at work. The analyses were restricted to 4259 respondents who worked at the 16th week. Potential confounders and effect modifiers were evaluated by stratification and multivariate analyses. RESULTS: After adjustment for confounders, women standing more than five hours per work day had an odds ratio (OR) for preterm delivery of 1.2 (95% CI 0.6 to 2.4) compared with women standing two hours or less. For walking, the OR was 1.4 (95% CI 0.7 to 2.5). Many women were unable to separate periods of standing from periods of walking; a combined measure of these two exposures was created to reflect exposure intensity. Women who reported more than five hours of both standing and walking had an adjusted OR of 3.3 (95% CI 1.4 to 8.0) compared with women who reported two hours or less on either of the exposures. No adverse effects were seen for lifting or other types of physical exertion. CONCLUSIONS: Our findings suggest that standing and walking at work during the second trimester may present a particular risk for preterm delivery, and workplace guidelines are justified. Further research is needed to address the specific mechanisms by which physical exertion, including standing and walking, might cause preterm delivery.