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.