Third trimester cervical length could help identify women at risk of spontaneous preterm birth

Women with spontaneous preterm birth had significantly shorter transvaginal cervical length (TVCL) measurements after 24⁺⁰ weeks of gestation compared with those who delivered at term, and a cutoff of 25.5 mm between 24⁺⁰ and 32⁺⁶ weeks may help identify women at elevated risk, according to a systematic review and meta-analysis published in the American Journal of Obstetrics and Gynecology.

Preterm birth, defined as delivery before 37⁺⁰ weeks of gestation, contributes to 75% of perinatal deaths and remains the predominant cause of neonatal death and death in children younger than 5 years. The worldwide preterm birth rate has stagnated at approximately 10%, with an estimated 13.4 million preterm infants born in 2020, according to the study investigators.

While TVCL measured in the second trimester is a well-established predictor of spontaneous preterm birth (SPTB), and interventions including vaginal progesterone and cervical cerclage are effective in women with a short second trimester TVCL (defined as less than 25 mm), the predictive role of TVCL beyond 24 weeks has remained unclear. Many women with SPTB do not have a short cervix in the second trimester, yet accelerated TVCL shortening can still occur after 24 weeks, creating a window for earlier detection and timely intervention to reduce SPTB.

Researchers conducted a systematic search of published literature in PubMed, MEDLINE, and the Cochrane Library through June 2025 with no language restriction. Cohort or cross-sectional studies reporting on TVCL measurements after 24⁺⁰ weeks in asymptomatic women with both SPTB and term births that allowed construction of 2×2 contingency tables were included. Studies involving symptomatic women, twin pregnancies, or transabdominal cervical length measurement were excluded, as were abstracts and studies with duplicated data.

Of 3641 published articles identified, 16 studies comprising 26,776 pregnancies met the inclusion criteria. All included studies excluded iatrogenic preterm birth. Study quality was evaluated using the Newcastle-Ottawa Scale, and mean differences in TVCL between SPTB and term populations were calculated using the inverse variance method with a random-effects model. The Youden index was used to identify the optimal TVCL cutoff.

How do gestational windows impact TVCL?

Women with SPTB had significantly shorter TVCL at 24⁺⁰ to 28⁺⁶ weeks compared with women with term birth, for both delivery before 37 weeks (mean difference: -5.47 mm; 95% CI: -7.52 to -3.43; P < .001) and delivery before 34 weeks (mean difference: -7.85 mm; 95% CI: -10.33 to -5.37; P < .001). Similar patterns were observed at 27⁺⁰ to 32⁺⁶ weeks for delivery before 37 weeks (mean difference: -4.41 mm; 95% CI: -6.45 to -2.36; P < .001) and before 34 weeks (mean difference: -7.75 mm; 95% CI: -10.55 to -4.94; P < .001).

TVCL at 31⁺⁰ to 36⁺⁶ weeks was comparable between the 2 groups for delivery before 37 weeks (mean difference: -4.71 mm; 95% CI: -10.13 to 0.72; P = .09). These findings were consistent across women with and without risk factors for SPTB, and irrespective of TVCL measurement method.

Predictive accuracy and optimal cutoff

A TVCL cutoff of 25.5 to 26.0 mm after 24⁺⁰ weeks demonstrated the highest positive likelihood ratio of 7.75 for predicting SPTB, with a pooled sensitivity of 0.74, specificity of 0.92, negative likelihood ratio of 0.31, and diagnostic odds ratio of 28.1. These metrics support the clinical utility of third trimester TVCL surveillance in asymptomatic women, particularly those who did not have a short cervix identified during second trimester screening.

Reference:

Cheung KW, Sin-Tung Au T, Phoebe Chao TY, Tin-Yan Seto, M. Predictive value of cervical length measured after 24 weeks for spontaneous preterm birth: systematic review and meta-analysis. American Journal of Obstetrics & Gynecology. Accessed July 1, 2026. https://www.ajog.org/article/S0002-9378(26)00015-3/fulltext