Risk of prematurity, low birthweight and pre-eclampsia in relation to working hours and physical activities: a systematic review

Background: Occupational activities are suspected of having an adverse impact on outcomes of pregnancy. Aim: To assess the evidence relating three major adverse outcomes ( preterm delivery, low birthweight ( LBW) and pre- eclampsia/ gestational hypertension) to five common occupational exposures ( prolonged working hours, shift work, lifting, standing and heavy physical workload). Methods: A systematic search of Medline and Embase ( 1966 - December 2005) using combinations of keywords and medical subject heading terms was conducted. For each relevant paper, standard details were abstracted that were then used to summarise the design features of studies, to rate their methodological quality ( completeness of reporting and potential for important bias or confounding) and to provide estimates of effect. For studies with similar definitions of exposure and outcome, pooled estimates of relative risk ( RR) in meta- analysis were calculated. Results: 53 reports were identified - 35 on preterm delivery, 34 on birth weight and 9 on pre- eclampsia or gestational hypertension. These included 21 cohort investigations. For pre- term delivery, extensive evidence relating to each of the exposures of interest was found. Findings were generally consistent and tended to rule out a more than moderate effect size ( RR.1.4). The larger and most complete studies were less positive, and pooled estimates of risk pointed to only modest or null effects. For small-for- gestational age, the position was similar, but the evidence base was more limited. For pre- eclampsia and gestational hypertension, it was too small to allow firm conclusions. Conclusions: The balance of evidence is not sufficiently compelling to justify mandatory restrictions on any of the activities considered in this review. However, given some uncertainties in the evidence base and the apparent absence of important beneficial effects, it may be prudent to advise against long working hours, prolonged standing and heavy physical work, particularly late in pregnancy. Our review identifies several priorities for future investigation.