Background The perinatal mortality and morbidity among twins vary by chorionicity. Although it is considered that monochorionicity is associated with an increased risk of preterm birth in twin pregnancies, no systematic review exists evaluating this association. Objectives This systematic review was undertaken to assess the association between preterm birth and chorionicity in twin pregnancies. Search strategy We searched the electronic databases from January 1990 to July 2019 without language restrictions. Selection criteria All studies on twin pregnancies where chorionicity and preterm birth were evaluated were included. Data collection and analysis Findings are reported as odds ratios with 95% confidence intervals. The estimates are pooled using random-effects meta-analysis. Main results From 13 156 citations, we included 39 studies (29 864 pregnancies). Monochorionicity was significantly associated with increased risk of preterm birth at ≤28, ≤32, ≤34 Conclusions Monochorionicity is significantly associated with preterm birth at all gestations.

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