Spontaneous preterm birth as a function of normal cervical length in low-risk women

Abstract

Objectives

To identify if there is an increased risk for spontaneous preterm birth (sPTB) across the continuum of measured, normal cervical lengths (CL) in low-risk women.

Methods

Retrospective cohort study of women with singleton pregnancies and no history of prior sPTB. Women were included if they underwent mid-trimester transvaginal CL measurement between February 2016 and August 2018 and had a measured, normal CL ≥25mm. Women were excluded for progesterone exposure, fetal anomalies, or an unmeasurable CL due to a poorly developed lower uterine segment. The primary study outcome was sPTB <37 weeks. Secondary outcomes included: sPTB <35 weeks, birth gestational age (GA), and the number of hospital evaluations for suspected preterm labor (PTL). Cervical length was considered in interval groups 25–29mm, 30–34mm, 35–39mm, 40–44mm, and ≥45mm. Outcomes were analyzed with χ² test of trend and as a continuum (linear models, logistic regression and ROC curve), where appropriate.

Results

985 women were included. The incidence of sPTB <37 weeks was 3.7%, with a mean birth GA of 38.7 ± 2.4 weeks. The odds of sPTB <37 weeks decreased with increasing cervical length, considered in 5 mm intervals (odds ratio = 0.67; 95% confidence interval 0.49–0.90) and an increasing birth GA of 1 additional day for each CL increase of 3mm (p = .0002). Conversely, sPTB <35 weeks (p = .49) and mean hospital evaluations for PTL (p = .26) were similar across groups. The ROC curve area-under-the-curve for sPTB <37 weeks of 0.64 showed poor predictive value.

Conclusions

Among women without a history of sPTB, there was an association of decreased risk of sPTB <37 weeks and advanced delivery GA with increasing, but normal-range CL measurements. However, the association was poor and was not associated with spontaneous preterm birth <35 weeks, or the number of hospital evaluations for PTL.

Keywords:

• Spontaneous preterm birth
• cervical length
• cervical length screening
• low-risk

Acknowledgements

The authors thank Mrs. Cherry Neely for her assistance in developing the database used for this project. This abstract was presented at the Society for Maternal-Fetal Medicine 40th Annual Pregnancy Meeting in Grapevine, TX, February 7, 2020.

Disclosure statement

No potential conflict of interest was reported by the author(s).