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.