OP11.04: Infrapubic intrapartum ultrasound (iiUS): sonographic landmarks and correlation with successful vacuum extraction

Abstract

Objective: Assessment of fetal head station is of importance before instrumental vaginal delivery. We have established a simple method of performing intrapartum infrapubic ultrasound (iiUS). Relating the fetal skull to the symphysis in laboring women during pushing and prior to vacuum extraction, we defined sonographic criteria for successful instrumental vaginal delivery. Methods: Sonographic assessment of the female pelvis in term pregnancies was performed in women with singleton pregnancies in the second stage of labor and after rupture of membranes. Anatomic landmarks were determined sonographically from sagittal infrapubic insonation with a curved array transducer. A 3D CT volume was used to correlate the normal anatomy of the female pelvis with these iiUS-derived landmarks. 20 women with singleton fetuses were studied immediately before vacuum extraction. Results: With a transducer placed infrapubically in a median sagittal orientation (‘infrapubic plane’), landmarks are: (1) the infrapubic line, placed perpendicular to the long axis of the pubic symphysis; (2) the widest fetal head diameter and its movement with regard to the infrapubic line during pushing; and (3) the ‘head direction’ as the angle between the long axis of the symphysis and a line perpendicular to the widest head diameter. CT reconstruction demonstrated the infrapubic line to be 3 cm cranial to the parallel interspinous plane. In 20 vacuum deliveries, the ‘head up’ sign (head pointing ventrally) and objective descent of the fetal head below the infrapubic line, both noted at the height of pushing, were associated with successful delivery. Lack of decent/lack of passage below the infrapubic line and horizontal or even downward head direction were poor prognostic signs. Conclusions: Intrapartum infrapubic ultrasound (iiUS) can provide information about the dynamics of the second stage of labor, head station and head direction. It may be used to assess prognosis for instrumental vaginal delivery.