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Chlamydia trachomatis and reproductive health: what can we learn from systematic reviews of observational studies?
  1. Nicola Low
  1. Institute of Social and Preventive Medicine, University of Bern, Bern 3012, Switzerland
  1. Correspondence to Professor Nicola Low, Institute of Social and Preventive Medicine, University of Bern, Bern 3012, Switzerland; nicola.low{at}

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We are used to reading that Chlamydia trachomatis (CT) infection in pregnancy is associated with a range of adverse outcomes, including preterm birth, low birth weight, spontaneous abortion, intrauterine growth restriction and neonatal death.1 2 These statements, often in the introduction to commentaries or research studies, are accompanied by references to general textbooks3 or studies in which strong associations have been found.4 5 But, how sure are we of these findings, why do we want to know and what can we learn?

Tang and colleagues have done a systematic review and meta-analysis,6 which substantially strengthen the completeness of the evidence base about the association between CT and adverse reproductive outcomes. They reviewed reports of 107 observational epidemiological studies (case–control, cohort or cross-sectional) published up to May 2018. Following good practice in systematic reviews of observational studies of aetiology,7 they defined the research question in a published protocol, stratified results by study design, provided unadjusted and confounder-adjusted estimates where available, assessed the risk of bias and examined reasons for heterogeneity. The 107 studies in this review reported on any of 12 outcomes of pregnancy or fertility. Tang et al’s extensive review is important for three main reasons. First, it is probably the largest systematic review of studies reporting on associations between C. trachomatis and reproductive outcomes. Second, the review finds empirical evidence that study setting, study design and analysis are associated with the strength of associations. Third, this review makes us think about what research studies are needed next.

In observational studies of aetiology, there are multiple sources of bias that affect their interpretation. In an earlier systematic review, Olson-Chen et al examined eight adverse outcomes of pregnancy, but not fertility, and included 56 studies published between 1970 and 2013,8 with some overlap in …

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