Chlamydia trachomatis and reproductive health: what can we learn from systematic reviews of observational studies?

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,

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