2.1| Study population
We retrospectively collected data for 1580 consecutive women with twin pregnancies in the Fujian Maternity and Child Health Hospital (with 1000 beds and an annual delivery number of more than 20,000) and the Fujian Provincial Hospital (with 2398 beds and an annual delivery number of more than 5000) from January 2017 to December 2019; 1065 of these cases were eligible for participation.
Subjects with any of the following conditions were excluded: uncertain pregnancy date, maternal or foetal indications for iatrogenic PTB at <32 weeks, twin birth weight <500 g, gestational age at birth <24 weeks, genetic or structural abnormalities of either foetus, stillbirth of one or two foetuses, monoamniotic or monochorionic twin pregnancy complicated by twin transfusion syndrome (TTTS) or twin anaemia–polycythaemia sequence (TAPS), placement of cervical cerclage, incomplete maternal data, or delivery at a medical centre other than ours. Women who gave birth before 24 weeks were excluded because, in most cases, these women are likely to represent a unique subgroup of women whose cervical changes would be detected very early and extremely obvious. Additionally, these women would not have had their cervical measurement at the indicated gestational stage in our study period, which is a major part of our research. As a result, we excluded 515 patients who met the exclusion criteria, and thus, 1065 patients met the inclusion criteria (Figure 1).
We assigned 764 samples collected from the Fujian Maternity and Child Health Hospital as the training group and 301 samples collected from the Fujian Provincial Hospital as the external validation group. All samples were reassessed by two obstetricians according to the inclusion and exclusion criteria (the flowchart showing the derivation of the development cohort and validation cohort is presented in Figure 1). The institutional ethical review boards of both included hospitals approved this retrospective analysis of anonymous data, and the requirement for informed consent was waived by the ethics review boards.