Can sonographic depiction of fetal head position prior to or at the onset of labor predict mode of delivery?

Given the irregular shape of the maternal pelvis and the relatively large dimensions of the fetal head at term, accommodation or adaptation of portions of the fetal head to various maternal pelvic planes is required for successful vaginal birth. Positional changes in the presenting part that are necessary to enable the fetus to navigate the pelvic canal are incorporated in the cardinal movements of labor and include: engagement, descent, flexion, internal rotation, extension, external rotation and expulsion1,2. During labor, these movements, although sequential, often exhibit considerable temporal overlap. For example, engagement is associated with concurrent flexion and descent of the fetal head. Additional adaptive measures include intrapartum moulding of the fetal head: changes in the fetal head shape due to external compressive forces, resulting in a shortened suboccipitobregmatic diameter and a lengthened mentovertical diameter1,2. Fetal descent in particular is essential for successful (non-operative) vaginal birth and hence serves as a major component in the clinical assessment of progress of labor. Lack of descent is often a result of malrotation of the fetal head, resulting in relative cephalopelvic disproportion. Fetal descent is evaluated clinically by digital vaginal assessment in relation to the maternal ischial spines in a highly subjective and frequently inaccurate manner1–3 and is often further compromised by the presence of moulding of the fetal head1,2. Ultrasound assessment of labor has been applied increasingly during recent years. Clinical parameters/cardinal movements of labor amenable to intrapartum sonographic assessment include fetal head position4–15, engagement16,17, descent and internal rotation18–25. Transperineal/translabial ultrasound has been utilized to enhance depiction of descent and internal rotation of the fetal head, and determine the following parameters: head direction, angle of the middle line, progression distance, head–perineum distance and angle of progression18–36. Intrapartum sonographic assessment of fetal descent is promising in that it potentially offers a more objective assessment compared with digital vaginal assessment3. Although clinical application remains to be determined, recently, Bamberg et al. reported real-time open magnetic resonance imaging, in the mid-sagittal plane, of extension of the fetal head in the late second stage of labor37. Of note, sonography may also depict moulding of the fetal head1,38. In addition, intrapartum sonographic findings of asynclitism (lateral deflection of the sagittal suture), which, when severe, is an additional etiology of cephalopelvic disproportion, have been reported recently1,2,39,40.