Predicting Asymptomatic Cervical Dilation in Pregnant Patients with Short Mid-trimester Cervical Length: A Secondary Analysis of a Randomized Controlled Trial

Introduction: Second trimester asymptomatic cervical dilation is a significant risk factor for early preterm birth. The objective of this study is to evaluate whether transvaginal ultrasound cervical length (CL) predicts asymptomatic cervical dilation on physical exam in women with short cervix (CL ≤25 mm) and no prior preterm birth.

Material And Methods: Secondary analysis of a randomized trial on pessary in asymptomatic singletons without prior preterm birth diagnosed with CL ≤25 mm between 18 and 23  weeks. Participants had transvaginal ultrasound and physical cervical exam and were randomized to pessary or no pessary with all patients with cervical length ≤20 mm offered vaginal progesterone. The primary outcome was to determine whether CL was predictive of asymptomatic physical cervical dilation ≥1 cm using receiver operating characteristic curve.

Results: In all, 119 women were included. Based on receiver operating characteristic curve, CL ≤11 mm was best predictive of cervical dilation ≥1 cm, with 75% sensitivity, 80% specificity, and area under the curve 0.73 (0.55-0.91), P = 0.009. Cervical length ≤11 mm had increased incidence of cervical dilation ≥1 cm on physical exam (30% vs 3%, odds ratio 12.29 (3.05-49.37) P < 0.001) with a negative predictive value of 97%. Patients with ≥1 cm dilation had increased preterm birth <37 weeks (75% vs 39%, P = 0.03) compared to those not dilated. Women with a CL ≤11 mm had increased preterm birth <37 weeks (77% vs 31%, P < 0.001), preterm birth <34 weeks (63% vs 22%, P < 0.001), and lower birthweight (1552 ± 1047 vs 2560 ± 1072 g, P < 0.001) compared to women with CL >11 mm.

Conclusions: Among singletons without prior preterm birth diagnosed with short cervix (≤25 mm), CL ≤11 mm may identify a subgroup of patients at high risk for asymptomatic cervical dilation and poor perinatal outcome. Physical exam should be considered and adjunctive preterm birth prevention measures should be studied in singletons with CL ≤11 mm.