National Center for Fetal Medicine, St. Olavs Hospital, Trondheim University Hospital, Trondheim, Norway; Department of Clinical and Molecular Medicine, Norwegian University of Science and Technology, Trondheim, Norway; Department of Obstetrics and Gynecology, Helse Stavanger, Stavanger University Hospital, Stavanger, Norway.
Department of Obstetrics and Gynecology, Landspitali University Hospital, Reykjavík, Iceland; Faculty of Medicine, University of Iceland, Reykjavík, Iceland.
Am J Obstet Gynecol. 2024 Mar;230(3S):S901-S912. doi: 10.1016/j.ajog.2021.08.030. Epub 2021 Aug 27.
Fetal head descent can be expressed as fetal station and engagement. Station is traditionally based on clinical vaginal examination of the distal part of the fetal skull and related to the level of the ischial spines. Engagement is based on a transabdominal examination of the proximal part of the fetal head above the pelvic inlet. Clinical examinations are subjective, and objective measurements of descent are warranted. Ultrasound is a feasible diagnostic tool in labor, and fetal lie, station, position, presentation, and attitude can be examined. This review presents an overview of fetal descent examined with ultrasound. Ultrasound was first introduced for examining fetal descent in 1977. The distance from the sacral tip to the fetal skull was measured with A-mode ultrasound, but more convenient transperineal methods have since been published. Of those, progression distance, angle of progression, and head-symphysis distance are examined in the sagittal plane, using the inferior part of the symphysis pubis as reference point. Head-perineum distance is measured in the frontal plane (transverse transperineal scan) as the shortest distance from perineum to the fetal skull, representing the remaining part of the birth canal for the fetus to pass. At high stations, the fetal head is directed downward, followed with a horizontal and then an upward direction when the fetus descends in the birth canal and deflexes the head. Head descent may be assessed transabdominally with ultrasound and measured as the suprapubic descent angle. Many observational studies have shown that fetal descent assessed with ultrasound can predict labor outcome before induction of labor, as an admission test, and during the first and second stage of labor. Labor progress can also be examined longitudinally. The International Society of Ultrasound in Obstetrics and Gynecology recommends using ultrasound in women with prolonged or arrested first or second stage of labor, when malpositions or malpresentations are suspected, and before an operative vaginal delivery. One single ultrasound parameter cannot tell for sure whether an instrumental delivery is going to be successful. Information about station and position is a prerequisite, but head direction, presentation, and attitude also should be considered.
胎儿头位下降可表示为胎先露和衔接。胎先露传统上基于对胎儿颅骨远端的临床阴道检查，与坐骨棘水平相关。衔接基于对骨盆入口上方胎儿头部近端的经腹检查。临床检查是主观的，需要进行下降的客观测量。超声是分娩中可行的诊断工具，可以检查胎儿的胎位、先露、位置、胎方位和胎势。本综述介绍了超声检查胎儿下降的概述。1977 年首次将超声用于检查胎儿下降。使用 A 型超声测量骶骨尖端至胎儿颅骨的距离，但此后发表了更方便的经会阴方法。其中，进展距离、进展角度和头耻骨距离在矢状面中进行检查，以耻骨联合下部为参考点。头会阴距离在额状面（经会阴横切扫描）中测量，即会阴至胎儿颅骨的最短距离，代表胎儿通过的产道的剩余部分。在高位时，胎儿头朝下，当胎儿在产道下降并使头部弯曲时，先转为水平方向，然后转为向上方向。可以通过超声经腹部评估胎儿下降情况，并测量耻骨上下降角。许多观察性研究表明，超声评估的胎儿下降情况可以预测引产前、入院时和第一产程和第二产程中的分娩结局。也可以进行纵向分娩进展检查。国际妇产科超声学会建议在第一产程或第二产程延长或停滞、怀疑胎位不正或胎方位不正以及在进行阴道助产分娩之前使用超声。单一的超声参数不能确定器械分娩是否成功。胎先露和胎位的信息是前提，但头位、胎方位和胎势也应考虑。
