General characteristics
A total of 2532 articles were identified, 57 were assessed with respect to their eligibility for inclusion, and 10 studies17-26 were deemed suitable to be included in this systematic review (Table S1; Figure S1). These 10 studies (including 304181 twin pregnancies) reported the occurrence of birthweight discordance in twin pregnancies complicated by GHD compared with those not. Eight of these included papers were cohort studies, whereas two were case-control studies17,19. Three of them were conducted in China19,24,26, two in the USA21,22, two in the UK17,25, two in Rome18,20 and one in Ireland23. Among the selected studies, four reported the intertwin birthweight discordance affected by GH17,18,24,26, three affected by CH17,21,22 and five affected by PE19-21,24,26 in detail. Only three of the included studies24-26 conducted stratification by twin chorionicity.
Quality assessment of the selected studies performed using NOS is presented in Table S2. Most of the studies scored well in terms of selection, comparability and outcome. The main weaknesses of these studies were their retrospective design, small sample size, lack of information on the follow-up outcome, different gestational ages at scan and heterogeneity in the cut‐off of birthweight discordance adopted.