Although the prevalence of a short cervix in a general obstetric population is low, recent analyses suggest that universal transvaginal (TV) ultrasound and treatment with vaginal progesterone are cost-effective. Assessment of the cervix by transabdominal (TA) ultrasound may be a useful initial screening test to detect short cervix diagnosed by TV ultrasound, thereby more efficiently identifying candidates for vaginal progesterone treatment. This prospective cohort study was designed to test the hypothesis that there is a threshold cervical length measured by TA ultrasound above which the risk of short cervical length (≤25 mm) measured by TV ultrasound is extremely low.
Transvaginal assessment of the cervix was offered for women between 18 weeks 0 days’ and 23 weeks 6 days’ gestation with a singleton fetus. Prevoid and postvoid TA cervical lengths were measured before TV ultrasound. Transabdominal images in the midsagittal plane were obtained for the cervical/vaginal interface, internal and external cervical os, outline of the cervical corpus, and the full length of the cervical canal. Demographic and medical data for each patient were collected from electronic medical records. The study was powered to detect a screen-positive rate at which the sensitivity would be 95% (95% confidence interval [CI], 90%–99%) for detecting women with short TV cervical length 25 mm or less. The cutoff of 25 mm was used for the primary outcome because cervical length up to 25 mm is associated with increased preterm birth risk. These women may benefit from interventions (eg, cerclage) if they have certain risk factors. Because vaginal progesterone is beneficial only for a cervical length of 20 mm or less, this cutpoint was a secondary outcome. Prevoid and postvoid TA cervical length measurements were compared to assess any significant difference in measurements based on bladder status. STATA 10.0 was used to perform statistical analysis Paired data were accounted for using appropriate statistical methods. P < 0.05 was considered significant.
Of 1349 patients meeting inclusion criteria, the final analysis included 1217 women. The TV cervical length was 25 mm or less in 76 patients (6.2%) and 20 mm or less in 34 (2.6%). The mean, prevoid, and postvoid TV cervical lengths were 36.1 (SD, 8.3) mm (95% CI, 35.6–36.6 mm), 34.6 (SD, 8.4) mm (95% CI, 34.1–35.1 mm), and 33.5 (SD, 8.4) mm (95% CI, 33.0–34.1 mm), respectively; prevoid and postvoid mean TA cervical lengths were significantly shorter than mean TV cervical length (P < 0.01). Prevoid TA ultrasound was 96.1% sensitive at a cutoff of 36 mm or less for detecting short cervix on TV ultrasound of 25 mm or less (95% CI, 90.0%–99.2%) with a specificity of 39.4% (95% CI, 36.7%–42.2%). The area under the receiver operating characteristic curve for TV cervical length 25 mm or less was 0.76. Prevoid TA ultrasound was 100% sensitive at a cutoff of 35 mm or less for detecting a short cervix on TV ultrasound 20 mm or less (95% CI, 89.1%–100.0%). Specificity was 40.8% (95% CI, 38.0%–43.7%) at this cutoff. The area under the receiver operating characteristic curve for TV cervical length 20 mm or less was 0.86. Body mass index, race, parity, insurance status, age, and prior cesarean delivery were not associated with a difference in the mean discrepancy between TA and TV cervical length.
These results indicate that TA screening may reduce the burden of universal cervical length screening in prenatal ultrasound units and support the clinical utility and feasibility of both universal TV ultrasound cervical length measurement and TA screening.
Maternal and Child Health Research Program, Department of Obstetrics and Gynecology, Hospital of the University of Pennsylvania, Philadelphia, PA