Birth size and gestational age in opposite-sex twins as compared to same-sex twins: An individual-based pooled analysis of 21 cohorts.

Further, both boy-girl and girl-girl pairs showed a lower risk (OR) of delivery before 37 weeks [0.92; 95% CI 0.86 to 0.98 (p = 0.007) and 0.88; 95% CI 0.82 to 0.95 (p = 0.001), respectively] and 34 weeks [0.89; 95% CI 0.81 to 0.99 (p = 0.024) and 0.80; 95% CI 0.71 to 0.90 (p = <0.001), respectively] than boy-boy pairs; a similar trend was observed for the risk before 31 weeks but did not reach statistical significance [0.88; 95% CI 0.73 to 1.06 (p = 0.165) and 0.82; 95% CI 0.66 to 1.02 (p = 0.078), respectively)].

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