The role of cervical length in women with threatened preterm labor: is it a valid predictor at any gestational age?

OBJECTIVE: To determine whether the predictive accuracy of sonographic cervical length (CL) for preterm delivery (PTD) in women with threatened preterm labor (PTL) is related to gestational age (GA) at presentation. STUDY DESIGN: A retrospective cohort study of all women with singleton pregnancies who presented with PTL at less than 34 + 0 weeks and underwent sonographic measurement of CL in a tertiary medical center between 2007 and 2012. The predictive accuracy of CL for PTD was stratified by GA at presentation. RESULTS: Overall, 1077 women who presented with PTL have had sonographic measurement of CL and met the study inclusion criteria. Of those, 223 (20.7%) presented at 24 + 0-26 + 6 weeks (group 1), 274 (25.4%) at 27 + 0-29 + 6 weeks (group 2), 283 (26.3%) at 30+0-31+6 weeks (group 3), and 297 (27.6%) at 32+0-33+6 weeks (group 4). The overall performance CL as a predictive test for PTD was similar in the 4 GA groups, as reflected by the similar degree of correlation between CL with the examination to delivery interval (r = 0.27, r = 0.26, r = 0.28, and r = 0.29, respectively, P = .8), the similar area under the receiver-operator characteristic curve (0.641-0.690, 0.631-0.698, 0.643-0.654, and 0.678-0.698, respectively, P = .7), and a similar decrease in the risk of PTD of 5-10% for each additional millimeter of CL. The optimal cutoff of CL, however, was affected by GA at presentation, so that a higher cutoff of CL was needed to achieve a target negative predictive value for delivery within 14 days from presentation for women who presented later in pregnancy. The optimal thresholds to maximize the negative predictive value for delivery within 14 days were 36 mm, 32.5 mm, 24mm and 20.5 mm for women who presented at 32 + 0 to 33 + 6 weeks, 30 + 0 to 31 + 6 weeks, 27 + 0 to 29 + 6 weeks and 24 + 0 to 26 + 6, respectively. CONCLUSION: CL has modest predictive accuracy in women with threatened PTL, regardless of GA at presentation. However, the optimal cutoff of CL for the purpose of clinical decision making in women with PTL needs to be adjusted based on GA at presentation.