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 preg
nancy. However, results from epidemiological studies on this topic are
ambiguous. The aim of this study was to evaluate the influence of sta
nding and walking at work in the second trimester on preterm delivery
in a population with a low frequency of other workplace hazards. Subje
cts and design A prospective cohort of 8711 women with singleton pregn
ancies was established during 1989 through 1991. Information was colle
cted during the 16th week of pregnancy about medical and obstetrical h
istory, general lifestyle factors and exposures at work. The analyses
were restricted to 4259 respondents who worked at the 16th week. Poten
tial 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 per
iods of walking; a combined measure of these two exposures was created
to reflect exposure intensity. Women who reported more than five hour
s of both standing and walking had an adjusted OR of 3.3 (95% CI 1.4 t
o 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 type
s of physical exertion. Conclusions Our findings suggest that standing
and walking at work during the second trimester may present a particu
lar risk for preterm delivery, and workplace guidelines are justified.
Further research is needed to address the specific mechanisms by whic
h physical exertion, including standing and walking might cause preter
m delivery.