584 Ultrasound measurement of the obstetric conjugate and the relative position of the internal cervical os

Abstract

The obstetric conjugate (OC) is the shortest anteroposterior pelvic diameter that the fetus must traverse during labor and delivery. Our objective was to assess the reliability of measurement of the OC using ultrasound during pregnancy. We also assessed the reliability of a novel measurement – the shortest distance between the OC and the internal cervical os (OCIO), hypothesized to correlate with labor success. All parturient women aged 18-50 who presented for anatomy ultrasound at our tertiary care medical center were included in the study. Multiple cine clips were obtained in which the pubic symphysis, sacral promontory, and internal cervical os could be seen. Two investigators independently reviewed the cine clips and obtained measurements of the OC and OCIO. Reviewers were blinded to the other’s measurements. Bland-Altman plots and interobserver intraclass correlation coefficients (ICC) were used to calculate interobserver reliability. A two-way mixed-effects model was assessed for calculating intra-class correlation coefficient among the two investigators. Agreement was considered excellent when the ICC was ≥ 0.9. Statistical analysis was performed using the Statistical Package for Social Science version 26.0 (SPSS Inc., Chicago, IL). 45 subjects were recruited for the study and measurements were possible in 37 cases. The mean ± S.D. age was 30 ± 5.3 years, gestational age 19.5 ± 1.6 weeks, and BMI 26.9 ± 6.1 kg/m2. The ICC for measurement of the OC was 0.96 (0.92 to 0.98) and 0.985 (0.97 to 0.99) for measurement of the OCIO. The Bland-Altman plots showed the 95% limits of agreement were -0.69 to 0.83 cm for the OC, and -0.56 to 0.91 for OCIO (Figure). The OC and the OCIO were measured with excellent interobserver reliability. Future studies will examine whether these measurements are associated with labor abnormalities.