Cervical length change as a predictor of preterm birth in symptomatic patients

Distinguishing between true and false preterm labor remains a challenge. The shortening in cervical length throughout a gestation has been theorized to be a possible predictor of spontaneous preterm birth. Although there are some studies evaluating cervical length shortening as a predictor of spontaneous preterm birth, it is not known whether the shortening in cervical length from an asymptomatic to symptomatic state, when a patient presents with preterm labor symptoms, is predictive of spontaneous preterm birth.

This study aimed to determine the utility of cervical length shortening from an asymptomatic time point (anatomic ultrasound) to when a patient presents with preterm labor symptoms as a predictor of spontaneous preterm birth.

A prospective cohort study was performed to evaluate the use of transvaginal cervical length assessment in symptomatic women in predicting spontaneous preterm birth from January 2013 to March 2015. Women with singleton gestations who presented to our institution between 22 0/7 weeks and 33 6/7 weeks of gestation with preterm labor symptoms were included in the overall cohort. This was a planned secondary analysis to evaluate the shortening in cervical length from an asymptomatic state (anatomic ultrasound) to a symptomatic state as a predictor of preterm birth. For this analysis, inclusion criteria were known delivery status, cervical length screening performed at anatomic ultrasound, and a valid cervical length measurement at the time of preterm labor symptoms. Women with preterm rupture of membranes, cervical dilation of >2 cm, or moderate to severe bleeding were excluded. Cervical length shortening was defined as a decrease in cervical length of >10 mm from anatomic ultrasound to the time of presentation with preterm labor symptoms. The outcome evaluated was spontaneous preterm birth before 37 weeks of gestation. Chi-square test and receiver operating characteristic curves were used to evaluate the data. Multivariable logistic regression was used to calculate the odds. Test characteristics of cervical length shortening of >10 mm were determined.

A total of 549 women were included in the original cohort, and 277 women were included in this secondary analysis. The overall rate of spontaneous preterm birth was 8.3%. There were 52 women (19%) with cervical length shortening of >10 mm. The rate of spontaneous preterm birth was significantly higher for those with cervical length shortening of >10 mm than those with cervical length shortening of ≤10 mm (21.2% vs 5.3%; P=.001). This higher risk of spontaneous preterm birth remained after adjusting for confounders including maternal age and previous spontaneous preterm birth (adjusted odds ratio, 4.71; 95% confidence interval, 1.84-12.09). Using cervical length shortening of >10 mm as a screening test had a sensitivity of 47.8%, a specificity of 83.9%, a positive predictive value of 21.2%, and a negative predictive value of 94.7%.

In women presenting with preterm labor symptoms, a cervical length that is >10 mm shorter from anatomic ultrasound is associated with an increased risk of spontaneous preterm birth.