Cervical Stiffness Index as predictor of preterm birth in women with threatened preterm labor

Objective: To determine the uterine cervix stiffness in patients presenting with threatened preterm labor (PTL) measured with an aspiration-based device (expressed as Cervical Stiffness Index − CSI) and to correlate CSI with pregnancy outcomes. Study design: Single-center, prospective, observational pilot study of patients with singleton pregnancies presenting with threatened PTL between 240/7and 336/7weeks. Patients underwent a physical examination, ultrasound examination, transvaginal ultrasound cervical length (CL) measurement and cervical stiffness assessment using the Pregnolia System, an aspiration-based device measuring the vacuum required to displace the tissue into the device tip by a fixed distance, a proxy for stiffness (CSI, mbar). Higher CSI indicates stiffer tissue. Results: Between April 2022 and August 2024, 100 patients with signs and symptoms of PTL were recruited. Thirty-five had a PTB, of which 7 delivered < 34 weeks, 5 within 14 days and 11 within 28 days from measurement. CSI and CL were significantly different between patients delivering at term and patients with a PTB. The Area under the Curve of CSI and CL for prediction of delivery ≤ 14 days from measurement were 0.979 (95 % CI, 0.952–1.000) and 0.744 (0.369–1.000), respectively; for delivery ≤ 28 days from measurement, 0.802 (0.612–0.993), and 0.764 (0.579–0.948), respectively; for PTB < 34 weeks, 0.873 (0.729–1.000) and 0.836 (0.563–1.000), respectively; and for PTB < 37 weeks 0.845 (0.763–0.926) and 0.779 (0.680–0.877), respectively. Conclusions: In patients presenting with threatened PTL, a softer cervix as measured by CSI was associated with an increased risk of preterm birth. These findings suggest that CSI has potential as a predictor of PTB in symptomatic patients, but require confirmation in larger, independent cohorts. Clinical trial registration: NCT05355649.