Characteristics of the development and external validation groups
Totally 1065 twin pregnancies were eligible for the study, of which 764 collected from the Fujian Maternity and Child Health Hospital were assigned to the training group, while 301 from the Fujian Provincial Hospital were into the external validation group (Figure 1). In the whole study population, the number of positive cases of SPTB at <28, 32, 34 and 37 weeks were 32(3.0%), 150(13.8%), 221(20.4%) and 639(58.9%), respectively.
Supplementary Table 1 lists the maternal demographic and clinical characteristics of the twin pregnancies in the training and external validation groups. There were no significant differences in these features between the training and validation groups (all P> 0.05), indicating that the characteristics of the training and external validation groups were similar and that subsequent external validation would be representative.