Objective: To determine whether an association exists between shift work and early reproductive outcomes.
Data sources: MEDLINE, Embase, and Web of Science were searched. Additional sources included Google Scholar, the Cochrane Library, online publications of national colleges, the ClinicalTrials.gov, and references of retrieved papers.
Methods of study selection: Included studies compared female shift workers (work outside 8:00 AM to 6:00 PM) with nonshift workers with menstrual disruption (cycles less than 25 days or greater than 31 days), infertility (time-to-pregnancy exceeding 12 months), or early spontaneous pregnancy loss (less than 25 weeks).
Tabulation, integration, and results: Two reviewers extracted adjusted and raw data. Random effect models were used to pool data weighting for the inverse of variance. Assessments of heterogeneity, bias, and subgroup analyses were performed. Sixteen independent cohorts from 15 studies (123,403 women) were subject to analysis. Shift workers had increased rates of menstrual disruption (16.05% [2,207/13,749] compared with 13.05% [7,561/57,932] [n=71.681, odds ratio {OR} 1.22, 95% confidence interval {CI} 1.15-1.29, I 0%]) and infertility (11.3% [529/4,668] compared with 9.9% [2,354/23,811] [OR 1.80, 95% CI 1.01-3.20, I 94%]) but not early spontaneous pregnancy loss (11.84% [939/7,931] compared with 12.11% [1,898/15,673] [n=23,604, OR 0.96, 95% CI 0.88-1.05, I 0%]). Night shifts were associated with increased early spontaneous pregnancy loss (n=13,018, OR 1.29, 95% CI 1.11-1.50, I 0%). Confounder adjustment led to persistent relationships between shift work and menstrual disruption (adjusted OR 1.15, 95% CI 1.01-1.31, I 70%) but not infertility (adjusted OR 1.11 95% CI 0.86-1.44, I 61%). The association between night shifts and early spontaneous pregnancy loss remained (adjusted OR 1.41 95% CI 1.22-1.63, I 0%).
Conclusion: This review provides evidence for an association between performing shift work and early reproductive outcomes, consistent with later pregnancy findings. However, there is currently insufficient evidence for clinicians to advise restricting shift work in women of reproductive age.