Statistical analyses
Data analysis was carried out using Review Manager (version 5.3) software. Heterogeneity between studies was explored using the Higgins I2 statistics. When there was substantial heterogeneity (I2 ≥ 50%), the random effect model was used to estimate the pooled risk ratio, otherwise the fixed effect model was used. Subgroup analyses were employed to evaluate the sources of heterogeneity, and stratification by twin chorionicity (DC and MC) was performed to exclude the impact of chorionicity on the outcome. The odds ratio (OR) and 95% confidence interval (CI) were the summary measures applied to assess each endpoint. Potential publication biases were statistically examined by Egger’s test and visually by funnel plots (Figure S2).