Anatomical identification of ischial spines applicable to intrapartum transperineal ultrasound based on magnetic resonance imaging of pregnant women

Intrapartum transperineal ultrasound is considered useful in judging fetal head descent; however, the inability to detect ischial spines on ultrasound images has been a drawback to its legitimacy. The current study aimed to determine the anatomical location of ischial spines, which can be directly applied to intrapartum transperineal ultrasound images. Based on magnetic resonance imaging (MRI) of 67 pregnant women at 33+2 [31+6-34+0] weeks gestation (median [interquartile range: IQR]), we calculated the angle between the pubic symphysis and the midpoint of ischial spines (midline symphysis-ischial spine angle; mSIA), which is theoretically equivalent to the angle of progression at fetal head station 0 on ITU, by determining spatial coordinates of pelvic landmarks and utilizing spatial vector analysis. Furthermore, we measured symphysis-ischial spine distance (SID), defined as the distance between the vertical plane passing the lower edge of the pubic symphysis and the plane that passes the ischial spines. As a result, mSIA was 109.6° [105.1–114.0] and SID 26.4 mm [19.8–30.7] (median, [IQR]). There was no correlation between mSIA or SID and maternal characteristics, including physique. We established a novel method to measure the components of the pelvic anatomy by analyzing the three-dimensional coordinates of MRI data and identified the anatomical location of ischial spines which can be applied to ultrasound images. Our results provide valuable evidence to enhance the reliability of intrapartum transperineal ultrasound in assessing fetal head descent by considering the location of ischial spines.