Change in cervical length after arrested preterm labor and risk of preterm birth

Abstract

Objective: To assess the association between preterm birth and cervical length after arrested preterm labor in high-risk pregnant women.

Methods: In this post-hoc analysis of a randomized clinical trial, transvaginal cervical length was measured in women whose contractions had ceased 48 h after admission for threatened preterm labor. At admission, women were defined as having a high risk of preterm birth based on a cervical length of < 15 mm or a cervical length of 15-30 mm with a positive fetal fibronectin test. Logistic regression analysis was used to investigate the association of cervical length measured at least 48 h after admission and of the change in cervical length between admission and at least 48 h later, with preterm birth before 34 weeks' gestation and delivery within 7 days after admission.

Results: A total of 164 women were included in the analysis. Women whose cervical length increased between admission for threatened preterm labor and 48 h later (32%; n = 53) were found to have a lower risk of preterm birth before 34 weeks compared with women whose cervical length did not change (adjusted odds ratio (aOR), 0.24 (95% CI, 0.09-0.69)). The risk in women with a decrease in cervical length between the two timepoints was not different from that in women with no change in cervical length (aOR, 1.45 (95% CI, 0.62-3.41)). Moreover, greater absolute cervical length after 48 h was associated with a lower risk of preterm birth before 34 weeks (aOR, 0.90 (95% CI, 0.84-0.96)) and delivery within 7 days after admission (aOR, 0.91 (95% CI, 0.82-1.02)). Sensitivity analysis in women randomized to receive no intervention showed comparable results.

Conclusion: Our study suggests that the risk of preterm birth before 34 weeks is lower in women whose cervical length increases between admission for threatened preterm labor and at least 48 h later when contractions had ceased compared with women in whom cervical length does not change or decreases. © 2021 The Authors. Ultrasound in Obstetrics & Gynecology published by John Wiley & Sons Ltd on behalf of International Society of Ultrasound in Obstetrics and Gynecology.

Keywords: arrested preterm labor; cervical length; change in cervical length; preterm birth; threatened preterm labor.

Figure 1

Numbers of women with preterm birth before 34 weeks () vs at or after 34 weeks () (a) and numbers with delivery within 7 days () vs after 7 days () following admission for threatened preterm labor (b), according to cervical length measured at least 48 h after admission.

Figure 2

Predicted risk, with 95% CI, of preterm birth before 34 weeks (red) and delivery within 7 days after admission for threatened preterm labor (blue), according to cervical length measured at least 48 h after admission. , Reference line at 10% predicted risk.

Figure 3

Predicted risk, with 95% CI, of preterm birth before 34 weeks (red) and delivery within 7 days after admission for threatened preterm labor (blue), according to change in cervical length between admission and at least 48 h later. , Reference line at 10% predicted risk.

Figure 4

Predicted risk, with 95% CI, of preterm birth (PTB) before 34 weeks, according to cervical length measured at least 48 h after admission for threatened preterm labor and category of change in cervical length between admission and at least 48 h later (increase in cervical length (blue), decrease in cervical length (red) and no change in cervical length (green)). , Reference line at 10% predicted risk.