title: Shift work, long working hours and preterm birth: a systematic review and meta-analysis.
abstract: OBJECTIVE
  Specific physical activities or working conditions are suspected for increasing the risk of preterm birth (PTB). The aim of this meta-analysis is to review and summarize the pre-existing evidence on the effect of shift work or long working hours on the risk of PTB.
  
  
  METHODS
  We conducted a systematic search in MEDLINE and EMBASE (1990-2013) for observational and intervention studies with original data. We only included articles that met our specific criteria for language, exposure, outcome, data collection and original data that were of at least of moderate quality. The data of the included studies were pooled.
  
  
  RESULTS
  Eight high-quality studies and eight moderate-quality studies were included in the meta-analysis. In these studies, no clear or statistically significant relationship between shift work and PTB was found. The summary estimate OR for performing shift work during pregnancy and the risk of PTB were 1.04 (95% CI 0.90-1.20). For long working hours during pregnancy, the summary estimate OR was 1.25 (95% CI 1.01-1.54), indicating a marginally statistically significant relationship but an only slightly elevated risk.
  
  
  CONCLUSIONS
  Although in many of the included studies a positive association between long working hours and PTB was seen this did reach only marginal statistical significance. In the studies included in this review, working in shifts or in night shifts during pregnancy was not significantly associated with an increased risk for PTB. For both risk factors, due to the lack of high-quality studies focusing on the risks per trimester, in particular the third trimester, a firm conclusion about an association cannot be stated.
authors.affiliation: Maastricht University Medical Centre, Department of Obstetrics and Gynaecology, PO Box 5800, Maastricht, 6202 AZ, The Netherlands, mvmelick@viecuri.nl.
authors.forename: M J G J
authors.initials: MJ
authors.lastname: van Melick
authors.forename: M D M
authors.initials: MD
authors.lastname: van Beukering
authors.forename: B W
authors.initials: BW
authors.lastname: Mol
authors.forename: M H W
authors.initials: MH
authors.lastname: Frings-Dresen
authors.forename: C T J
authors.initials: CT
authors.lastname: Hulshof
mesh_terms: D005260:Female; D006801:Humans; D009784:Occupational Diseases / Q000209:etiology / Q000503:physiopathology; D011247:Pregnancy; D047928:Premature Birth / Q000209:etiology*; D014940:Work Schedule Tolerance / Q000502:physiology*