Objectives Miscarriage is a prevalent adverse pregnancy outcome that has been linked to ergonomic risk factors at work in numerous studies but no scientific consensus has emerged. In order to improve the impact of occupational health counselling of pregnant women the objective of this study was to quantify the risk of miscarriage according to prevalent work activities and to discuss causal inference based upon all available scientific data.
Methods A search in Medline and EMBASE 1966 - 2012 identified 29 primary papers reporting the relative risk (RR) of miscarriage according to work at night, long working hours, heavy lifting or prolonged standing. Following an assessment of completeness of reporting, confounding and bias, each risk estimate was characterised as more or less likely to be biased. Studies with equivalent measures of exposure were pooled to obtain a weighted common risk estimate. Sensitivity analyses excluded studies most likely to be biased.
Results Working fixed nights was associated with a moderately increased risk of miscarriage [pooled RR 1.51 (95% CI 1.27–1.78, n = 5)], while working for more than 40–52 hours weekly, lifting>100 kg/day and standing > 6–8 hours/day were associated with small risk increments: the pooled RRs ranging from 1.16 (prolonged standing, number of risk estimates 6) to 1.33 (working hours, number of risk estimates 9). Most RRs tended to become smaller and statistically non-significant when analyses were restricted to higher quality studies.
Conclusions These largely reassuring findings do not provide a case for mandatory restrictions in relation to working fixed night shifts, long working hours, occupational lifting and prolonged standing. Considering the limited evidence base, however, it seems prudent to advise women against work entailing high levels of these exposures.