(350) Second trimester ultrasound measurements of the pelvis to predict mode of delivery in primiparous patients

We hypothesized that second trimester, ultrasound-derived measurements of the obstetric conjugate (OC) and a novel measurement- the distance between the OC and the internal cervical os (OCIO) (Figure 1) - would predict successful vaginal deliveries in primiparous patients

Study Design:

This was a prospective observational study. Patients presenting for an anatomy scan at 18 – 22 weeks at our tertiary care medical center were consented. Three cine clips were obtained in which the pubic symphysis, sacral promontory, and internal cervical os were visualized. Two investigators independently reviewed the cine clips and performed offline measurements of the OC and OCIO. The agreement between the two investigators was measured using intraclass correlation coefficient (ICC). The mode of delivery was subsequently recorded for patients who labored and delivered at term.

Results:

A total of 99 primiparous patients were recruited. Of these, the anatomical landmarks were not seen in 7 (7.1%) patients. For the remaining 92 patients, the ICC for the OC and OCIO was 0.85  and 0.80 respectively, indicating good inter-rater agreement. The mean OC ± S.D was 12.2 ± 0.84 cm. The mean OCIO was 1.4 ± 1.9 cm. For delivery information, an additional 18 patients were excluded (delivery outcome data was not available (n=6), cesarean section was planned (n=6), and delivery was prior to 37 weeks (n=6)). Thus, the mode of delivery was recorded in 74 patients delivering at term. The OC was not associated with subsequent mode of delivery (p = 0.8). However, the OCIO was a significant predictor of the mode of delivery (p=.002, Figure 2). The area under the ROC curve was 0.73. When the internal cervical os was positioned above the OC, the relative risk of cesarean section was 2.2 (p=0.02).

Conclusion:

The second trimester distance of the OCIO, but not the OC, was significantly associated with mode of delivery at term. In anatomical terms, when the internal cervical os was higher in the pelvis in the midtrimester, the likelihood of cesarean section was significantly higher.