Variables of interest
We scrutinized the sonographic reports in the electronic database of the hospital to identify patients with a short cervix. The relevant information was then collected from the electronic medical records using their hospital identity of each patient. Throughout the study period, no significant changes was made to the electronic medical database.
We collected the demographic characteristics, obstetric data, risk history, and preexisting comorbidities. The demographic characteristics included were: maternal age at conception and body mass index (BMI) before pregnancy. For robust clinical assessment, BMI was classified into “normal” (18.5≤ BMI ≤23.9kg/m2), “overweight” (24≤ BMI ≤27.9kg/m2) and “obesity” (BMI ≥28kg/m2) groups12. None of the women was a smoker or drinker, thus lifestyle factors (e.g., smoking and drinking) were not analyzed in this study. In the obstetrics data, gestational age at diagnosis of short cervix, cervical length and amniotic fluid sludge were collected when the patient’s short cervix was first detected. Risk history was defined as having a previous short cervical length or PTB (e.g., induced abortion, intrauterine operation frequency (except abortion) and history of preterm birth). Autoimmune diseases included in the preexisting comorbidities was defined as systemic lupus erythematosus (SLE) or antiphospholipid syndrome (APS) that was found to be related to PTB13. To explore the relationship between some pregnancy complications and PTB among participants, we collected data such as polyhydramnios, gestational diabetes mellitus, and blood pressure state.