Predictive value of short cervix in early third trimester for preterm birth in women with normal mid-trimester cervical length.

Background: Short cervical length in the mid-trimester is a powerful predictor of preterm birth (PTB). However, clinical significance of cervical length in the third trimester for predicting PTB has not been established yet.
Objective: To examine the predictive role of a shortened cervix in the third trimester for PTB in women who had a normal cervical length in the second trimester STUDY DESIGN: This retrospective cohort study included women who underwent cervical length measured at least once in both the second trimester (16+0 weeks to 27+6weeks) and the early third trimester (28+0 weeks to 33+6 weeks). Women with short cervical length in the second trimester, those with multiple pregnancies, those who underwent cerclage operation, and those who had iatrogenic PTB were excluded. The study population was divided into two groups based on cervical length in the third trimester: a short cervix (≤25 mm) group and a control group (>25 mm). Rates of PTB (<37 weeks) were compared between two groups. Predictive performances of cervical length in the third trimester for PTB were assessed.
Results: Women with a short cervical length at 28+0 to 33+6 weeks accounted for 12.6% (n=717) of the total study population (n=5682). PTB rate was 9.5% in the short cervix group, which was significantly higher than that (3.2%) in the control group (P<.001). The adjusted odds ratio for short cervical length on PTB was 2.73 (95% CI: 1.96-3.79). A short cervix in the third trimester had a sensitivity of 30.1%, a specificity of 88.1%, a positive predictive value of 9.5%, and a negative predictive value (NPV) of 96.8% in predicting PTB. The addition of third-trimester cervical length to the predictive model significantly increased the area under the curve from 0.64 (95% CI: 0.60-0.68) to 0.67 (95% CI: 0.63-0.71) (P=.002), demonstrating improved predictive performance.
Conclusion: Approximately 13% of women with normal cervical length in the mid-trimester had a short cervical length after 28 weeks, which increased the risk of PTB. The high specificity and NPV of third-trimester cervical length measurements underscore their critical utility in identifying women at low risk for preterm delivery.
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