Late Cervical Shortening following Midtrimester Screening: Associations with Obstetric History and Risk of Preterm Birth.

The objective of this study was to evaluate the clinical significance of late-onset cervical shortening diagnosed between 24.0 and 34.0 weeks of gestation following normal midtrimester screening and to assess its association with spontaneous preterm birth (sPTB) and adverse neonatal outcomes. We also examined whether obstetric history modifies this risk. This was a retrospective cohort study. Participants/Materials: A total of 500 singleton pregnancies with normal midtrimester cervical length (CL) who were subsequently diagnosed with cervical shortening (<25 mm) between 24.0 and 34.0 weeks of gestation were included in the study. This study was conducted at a tertiary, university-affiliated medical center (Galilee Medical Center, Israel) between March 2020 and May 2025. Patients with asymptomatic late cervical shortening were categorized into three groups: <10 mm, 11-15 mm, and 16-25 mm. CL measurements were further stratified by gestational age at diagnosis (24.0-27.6, 28.0-31.6, and 32.0-34.0 weeks). The primary outcome was sPTB <37 weeks. Secondary outcomes included sPTB <34 and <32 weeks and neonatal morbidity. Multivariable logistic regression was used to estimate adjusted odds ratios. Receiver operating characteristic (ROC) curve analysis was performed to determine gestational age-specific CL thresholds for predicting sPTB. A total of 500 patients were included: 24/500 (4.8%) with CL <10 mm, 80/500 (16.0%) with CL 11-15 mm, and 396/500 (79.2%) with CL 16-25 mm. Baseline characteristics were comparable across groups. Rates of sPTB <37 weeks were 18/24 (75.0%) in patients with CL <10 mm, 42/80 (52.5%) in those with CL 11-15 mm, and 99/396 (25.0%) in those with CL 16-25 mm (p < 0.001). Similarly, sPTB <34 weeks occurred in 14/24 (58.3%), 20/80 (25.0%), and 33/396 (8.3%), and sPTB <32 weeks in 11/24 (45.8%), 12/80 (15.0%), and 20/396 (5.1%) (all p < 0.001). When stratified by CL groups, sPTB rates did not differ according to obstetric history among patients with CL <10 mm and 11-15 mm. In contrast, among patients with CL 21-25 mm, sPTB occurred in 7/26 (26.9%) of those with a prior sPTB, 17/78 (21.8%) of nulliparas, and 10/78 (12.8%) of those with a prior term birth (p = 0.041). ROC analysis identified gestational age-specific thresholds for predicting sPTB <37 weeks: 22 mm at 24.0-27.6 weeks (area under the curve [AUC] 0.69, p < 0.001), 20 mm at 28.0-31.6 weeks (AUC 0.62, p = 0.002), and 18 mm at 32.0-34.0 weeks (AUC 0.72, p < 0.001). Correspondingly, sPTB rates were higher below these cutoffs: 49.1% vs. 18.2% (p < 0.001), 49.1% vs. 23.2% (p < 0.001), and 43.9% vs. 18.8% (p < 0.001), respectively. The retrospective design may introduce selection and information bias. Measurements were performed by multiple clinicians, potentially leading to interobserver variability. Residual confounding cannot be excluded. Late-onset cervical shortening after normal midtrimester screening is strongly associated with sPTB and adverse neonatal outcomes. The degree of shortening and obstetric history significantly influence risk, particularly in patients with moderate late shortening. Gestational age-specific CL thresholds may enhance late pregnancy risk stratification and support individualized surveillance and management strategies.