Study design and selection of participants
We retrospectively analyzed data of a cohort of pregnant women with a short cervix. All women had their delivery at Peking University First Hospital in Beijing, China, from January 1, 2017 to January 1, 2018. In our center, the pregnancies underwent transvaginal ultrasonography throughout the gestational period and the cervical length is routinely measured. A short cervix was defined as cervical length < 25mm under transvaginal ultrasonography. In women showing signs of miscarriage such as bleeding and contraction, we also recommended transvaginal ultrasonography. Patients with triple gestations, who underwent induced abortions due to personal reasons and patients whose short cervix was first detected after 36 weeks were excluded from this study. Women had received a preventive cervical ligation treatment were also excluded from this study. In the analysis of PTB < 34 weeks, pregnancies whose short cervix was first detected after 33 weeks were excluded.(Figure 1) The sonographers who performed the scans were extensively trained and passed a practical examination administered by an expert organization of Beijing.