The use of cervical sonography to differentiate true from false labor in term patients presenting for labor check

Abstract

Background: Cervical length by transvaginal ultrasound to predict preterm labor is widely used in clinical practice. Virtually no data exist on cervical length measurement to differentiate true from false labor in term patients who present for labor check. False-positive diagnosis of true labor at term may lead to unnecessary hospital admissions, obstetrical interventions, resource utilization, and cost.

Objective: We sought to determine if cervical length by transvaginal ultrasound can differentiate true from false labor in term patients presenting for labor check.

Study design: This is a prospective observational study of women presenting to labor and delivery with labor symptoms at 37-42 weeks, singleton cephalic gestation, regular uterine contractions (≥4/20 min), intact membranes, and cervix ≤4 cm dilated and ≤80% effaced. Those patients with placenta previa and indications for immediate delivery were excluded. The shortest best cervical length of 3 collected images was used for analysis. Providers managing labor were blinded to the cervical length. True labor was defined as spontaneous rupture of membranes or spontaneous cervical dilation ≥4 cm and ≥80% effaced within 24 hours of cervical length measurement. In the absence of these outcomes, labor status was determined as false labor. Receiver operating characteristic curves were generated to assess the predictive ability of cervical length to differentiate true from false labor and were analyzed separately for primiparous and multiparous patients. The diagnostic accuracies of various cervical length cutoffs were determined. The relationship of cervical length and time to delivery was also analyzed including both use and nonuse of oxytocin.

Results: In all, 77 patients were included in the study; the prevalence of true labor was 58.4% (45/77). Patients who were in true labor had shorter cervical length as compared to those in false labor: median 1.3 cm (range 0.5-4.1) vs 2.4 cm (range 1.0-5.0), respectively (P < .001). The area under the receiver operating characteristic curve for primiparous patients was 0.88 (P < .001) and for multiparous patients was 0.76 (P < .01), both demonstrating good correlation. The area under the receiver operating characteristic curves were not significantly different between primiparous and multiparous (P = .23). The area under the receiver operating characteristic curve for primiparous and multiparous patients combined was 0.8 (P < .0001), indicating a good overall correlation between cervical length and its ability to differentiate true from false labor. Overall, a cervical length cutoff of ≤1.5 cm to predict true labor had the highest specificity (81%), positive predictive value (83%), and positive likelihood ratio (4.2). There were no differences in cervical length prediction between primiparous and multiparous patients. Cervical length was positively correlated with time to delivery, regardless of the use of oxytocin.

Conclusion: In differentiating true from false labor in term patients who present for labor check, a cervical length of ≤1.5 cm was the most clinically optimal cutoff with the lowest false positive rate-due to its highest specificity-and highest positive predictive value and positive likelihood ratios. Its use to decide admission in patients at term with labor symptoms may prevent unnecessary admissions, obstetrical interventions, resource utilization, and cost.

Keywords: cervical length; cervical sonography; false labor; labor check; term gestation; true labor.