Angle of progression measurements of fetal head at term: a systematic comparison between open magnetic resonance imaging and transperineal ultrasound

AbstractAim:Fetal skull molding is important for the adaptation of the head to the birth canal during vaginal delivery. Importantly, the fetal head must rotate around the maternal symphysis pubis. The goals of this analysis were to observe a human birth in real-time using an open magnetic resonance imaging (MRI) scanner and describe the fetal head configuration during expulsion.Methods:Real-time cinematic MRI series (TSE single-shot sequence, TR 1600 ms, TE 150 ms) were acquired from the midsagittal plane of the maternal pelvis during the active second stage of labor at 37 weeks of gestation. Frame-by-frame analyses were performed to measure the frontooccipital diameter (FOD) and distance from the vertex to the base of the fetal skull.Results:During vaginal delivery in an occiput anterior position, the initial FOD was 10.3 cm. When expulsion began, the fetal skull was deformed and elongated, with the FOD increasing to 10.8 cm and 11.2 cm at crowning. In contrast, the distance from the vertex to the base of the skull was reduced from 6.4 cm to 5.6 cm at expulsion.Conclusions:Fetal head molding is the change in the fetal head due to the forces of labor. The biomechanics of this process are poorly understood. Our visualization of the normal mechanism of late second-stage labor shows that MRI technology can for the first time help define the changes in the diameters of the fetal head during active labor.

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Anatomical Identification of Ischial Spines Applicable to Intrapartum Transperineal Ultrasound Based on Magnetic Resonance Imaging of Pregnant Women

10.21203/rs.3.rs-269362/v1 ◽

2021 ◽

Author(s):

Eriko Yano ◽

Takayuki Iriyama ◽

Shouhei Hanaoka ◽

Seisuke Sayama ◽

Mari Ichinose ◽

...

Keyword(s):

Magnetic Resonance Imaging ◽

Magnetic Resonance ◽

Pregnant Women ◽

Vertical Plane ◽

Pubic Symphysis ◽

Transperineal Ultrasound ◽

Anatomical Location ◽

Ischial Spine ◽

Resonance Imaging ◽

Fetal Head

Abstract Intrapartum transperineal ultrasound (ITU) is considered useful in judging fetal head descent; however, the inability to detect ischial spines on ITU 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 ITU. Based on magnetic resonance imaging 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. Our results provide valuable evidence to enhance the reliability of ITU in assessing fetal head descent by considering the location of ischial spines.

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