2.2| Data collection
Data were extracted from the medical charts. Demographic characteristics included maternal age, prepregnancy body mass index (prepregnancy BMI), nulliparity, history of previous cervical surgery, history of smoking, clinical data including validation of gestational age by first trimester ultrasound, chorionicity, history of previous preterm or late abortion, complications during pregnancy, use of assisted reproductive technology, cervical length (20-24 weeks) and cervical funneling, gestational age at delivery.
Gestational age was calculated from the last menstrual period (LMP) and confirmed by the foetal crown-rump length measurement at the first trimester ultrasonic scan. If a discrepancy of more than 7 days was observed, the sonographic gestational age was followed. Chorionicity was confirmed by identifying lambda and T signs with ultrasound imaging between 11+0 and 13+6 weeks of gestation.21
All patients underwent transvaginal cervical length (TVCL) measurements between 20-24 weeks prior to viability and at a gestational age when the optimal image of the cervix was relatively easy to capture. The TVCL measurements of all subjects were performed by experienced sonographers at our ultrasound units. The ultrasound assessment was performed to measure the length of the cervical canal from the internal os to the external os and observe whether the cervical funneling appears with patients in the lithotomy position with an empty bladder. The measurement was repeated under fundal pressure or the Valsalva manoeuvre unless severe cervical shortening was observed.16 In addition, all the ultrasonic reports were reviewed in detail for information on the presence, shape and length of cervical funneling; the length of the closed part of the cervix; and changes in cervical funneling and the closed part in response to increased abdominal pressure.23