Traditionally, descent or station of the fetal head in the birth canal is assessed by digital examination. The most common technique is the description of the level of the fetal head in relation to the plane of the ischial spines.

Abstract: Traditionally, descent or station of the fetal head in the birth canal is assessed by digital examination. The most common technique is the description of the level of the fetal head in relation to the plane of the ischial spines. The fetal head station is considered at zero when the lowermost part of the fetal head is at the level of ischial spines. The long axis of the birth canal above and below the spine, is expressed in cm. Current evidence shows that, in the evaluation of fetal head station, vaginal digital assessment is poorly reliable. Ultrasound may be a useful tool in the assessment of fetal head descent in the birth canal thanks to its high level of reproducibility. Many parameters have been proposed to determine fetal head station in the birth canal using the transperineal approach both in the sagittal and axial plane. Currently, there is no consensus regarding which is the best sonographic parameter to assess fetal head station. The most studied parameters are the angle of progression (AoP), the fetal head direction, the head-symphysis distance (HSD), the head perineum distance (HPD), and the midline angle (MLA). The main guidelines on intrapartum ultrasound do not recommend the routine use of ultrasound in the labor ward, however, they promote the use of intrapartum ultrasound as a support to clinical evaluation in particular circumstances. Examples of clinical situations in which transperineal ultrasound may be useful in the assessment of fetal head descent in the birth canal include cases of slow progression or arrest of labor in the first and second stage of labor and for evaluation prior to performing operative vaginal delivery.

Keywords: Fetal head descent, fetal head station, intrapartum ultrasound, transabdominal ultrasound, transperineal ultrasound

Authors: Elena Brunelli1,2, MD, Aly Youssef1,2, MD, PhD

1. Obstetric Unit, IRCCS Azienda Ospedaliero-Universitaria di Bologna, Italy
2. Alma Mater Studiorum - Università di Bologna, Italy

Reviewers: Karen Fung-Kee-Fung

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Introduction

The evaluation of the fetal head descent in the birth canal is of paramount importance in the labor ward. The level or station of the fetal head (or in general, of the presenting part) in the birth canal is generally described in relationship to the ischial spines. The station is considered at zero (0) when the lowermost part of the fetal head is at the level of ischial spines, which are halfway between the pelvic inlet and outlet. The long axis of the birth canal above and below the spines is expressed in cm. Thus, when the fetal head descends from the inlet toward the ischial spines it passes from a station of -5 cm to -1 cm. As the fetal head descents below the spines, it passes from +1 cm to +5 cm; the latter corresponds to the visibility of the fetal head at the introitus. When the lowermost part of the fetal head is at 0 station, the fetal head is considered engaged, that it means that the biparietal plane has passed through the pelvic inlet1. Current evidence shows that the clinical assessment of the fetal head station by vaginal examination is poorly reliable2. Many ultrasound parameters have been proposed to determine fetal head station in the birth canal, with high reproducibility.3-5 

Sonographic technique

The sonographic assessment of fetal head station is performed by transperineal ultrasound in the midsagittal plane or axial plane. For the midsagittal plane, the transducer, covered by a sterile glove, is placed longitudinally between the two labia majora. The woman is positioned in semi-recumbent position, with the back elevated at 45 degrees and legs flexed at 45 degrees at the hips and the knees flexed at 90 degrees, with empty bladder. The midsagittal plane allows to depict some landmarks: the pubic symphysis (ideally displayed in a horizontal position) and the fetal head6. The ischial spines cannot be seen in this view; however, it was demonstrated that a fixed relationship exists between the interischial plane and the pubic symphysis. The infrapubic line (a line drawn from the caudal part of the pubis, perpendicular to its long axis, extending to the dorsal part of the birth canal) has been shown to be 3 cm above the interischial plane7, 8. For the axial plane, it is sufficient to rotate the transducer of 90° to obtain the correct imaging. 

Several parameters have been proposed to evaluate the fetal head station6, 9. Different approaches have been suggested, but there is no consensus regarding which is the best sonographic parameter in the evaluation of fetal head station. Hereafter we discuss the most studied sonographic methods in the evaluation of fetal head descent. 

On transperineal, midsagittal imaging the most studied parameters are the angle of progression, the fetal head direction, and the head-symphysis distance.

On transperineal imaging in the axial plane the most studied parameters are the head perineum distance and the midline angle, which evaluate fetal head station and rotation, respectively.

Angle of progression (AoP)

The angle of progression (or angle of fetal head descent) is the angle formed by a line drawn along the long axis of the pubic symphysis and a line drawn from the lowest part of the pubis to the deepest part of the fetal skull. Many studies have demonstrated that AoP is an objective, reproducible and easily performed technique to assess the fetal head station10, 11. It was estimated that an AoP of 120° correlates with a fetal head at 0 station and indicates fetal head engagement10. Tutschek et al. created a reference chart for the conversion between AoP and transperineal ultrasound head station, calculated using a formula obtained by regression of head station over angle of progression12.

Fetal head direction

The head direction is the angle formed by a line drawn along the major axis of the pubic symphysis and the fetal head midline7. The fetal head direction is classified in three categories: head down, if the angle is <0°, horizontal head, if the angle is between 0 and 30°, and head up, if the angle is > 30°. Henrich et al. in a prospective study published in 2006, demonstrated that fetal head direction changed during fetal head descent, from downward to horizontal to upward7.  

Head-symphysis distanceĀ 

The head-symphysis distance (HSD) is the distance between the lowest edge of the maternal symphysis pubis and the nearest point of the fetal head, along the infrapubic line13. The parameter is considered an indirect marker of fetal head descent, based on the clinical practice to evaluate the palpable space between the fetal skull and maternal pubis in the determination of the fetal head station. The HSD can be measured only at stations ≥- 3cm, which corresponds with stations below the infrapubic line. As the fetal head becomes more engaged in the birth canal, HSD values tend to decrease.

Head-perineum distance

The head-perineum distance (HPD) is measured in the axial transperineal plane, with the soft tissue compressed against the pubic bone and angled until the fetal skull is well depicted. HPD is measured as the shortest distance from the fetal skull to the perineum. Different studies have demonstrated that when the fetal head station is at 0 level the HPD is between 35 and 38 mm7, 14-16. 

Midline angle

The midline angle (MLA) is measured in the axial plane as the angle formed between the anteroposterior axis of the maternal pelvis and the echogenic line between the fetal cerebral hemispheres17. In fetuses in non-occiput posterior position, a significant correlation between fetal head descent and rotation has been demonstrated, represented by MLA 17. It has also been demonstrated that a rotation < 45° corresponds to a fetal head station of ≥+3 cm in 84% of cases and a rotation ≥45° corresponds to a fetal head station of ≤2 cm in 99% of cases17.

Clinical applications of sonographic fetal head stationĀ 

The ISUOG guidelines on intrapartum ultrasound and the RCOG guidelines on assisted vaginal delivery do not endorse the routine use of ultrasound in the labor ward6, 18, 19. However, the guidelines state that intrapartum ultrasound may be useful, as a support to clinical evaluation, in some circumstances (i.e. slow progression or arrest of labor in the first and second stage of labor and before performing an operative vaginal delivery). 

1.    Slow progression or arrest of labor in the first and second stage of labor
The first stage of labor is from the onset of labor to full cervical dilatation, and it is divided into a latent phase and an active phase, characterized by cervical effacement and rapid cervical dilatation.  The second stage of labor is marked from full cervical dilatation to the birth of the baby20. It has been suggested that the active first stage of labor begins at 6 cm of cervical dilatation21. However, the definitions related to the duration of first stage of labor are quite different in literature and guidelines, thus the definition of prolonged first stage of labor also differs20. Relatively to the second stage of labor more precise indications were released. The American College of Obstetricians and Gynecologist (ACOG) has defined prolonged second stage of labor as failure of fetal head descent for 1-2 hours in multiparous women and 2-3 hours for nulliparous women, depending on whether administration of epidural analgesia has occurred22. As the ISUOG guideline has stated, ultrasound may be useful in the management of arrested labor in the first and second stage of labor, helping to understand which women are at higher risk of operative delivery.

Relative to the first stage of labor, it has been demonstrated that an HPD < 40 mm and AoP > 110° in cases of prolonged first stage of labor are correlated with a higher chance of vaginal delivery23, 24. Another study aimed to develop models for the prediction of cesarean section in cases of failure to progress in a population of women undergoing induction of labor. The model included the ultrasound evaluation of fetal head station using AoP and demonstrated that the wider the AoP, the lowest the risk of cesarean delivery. The ultrasound model (which combined measurement of AoP with maternal height, parity, estimated fetal weight) showed an area under the receiver operating curve (ROC) of 80%25.

Relative to the second stage of labor, a prospective study including 62 nulliparous and multiparous women in occiput anterior position demonstrated that in cases of prolonged second stage of labor, an upward direction of the fetal head can be highly predictive of spontaneous vaginal delivery26

2.    Ultrasound before instrumental vaginal delivery
Ultrasound in labor ward may be useful also to ascertain fetal head position and station prior to an operative vaginal delivery, to ensure a more precise placement of vacuum cup closer to the flexion point and to more objectively evaluate the fetal head level in comparison to digital examination27. A 2018 observational study demonstrated that the best cutoffs values for successful operative deliveries was 138.7° at rest (sensitivity 86.2%, specificity 51.9%) and 160.9° during maternal pushing (sensitivity 87.1%, specificity 74.1%). Furthermore, a recent systematic review and meta-analysis estimated that with an AoP measured during maternal pushing > 160.9°, the probability of uncomplicated operative vaginal delivery is 97%28. Finally, a multicenter prospective cohort study conducted prior to performing operative delivery in nulliparous women with prolonged second stage of labor demonstrated that only 2.2% (3/138) fetuses in occiput anterior position with a head-perineum distance ≤35 mm was delivered by Caesarean Section vs. 35.3% (6/17) with non-occiput anterior position and head-perineum distance >35 mm15

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The article should be cited as: Brunelli E, Youssef A: The use of ultrasound in the assessment of fetal head descent in labor, Visual Encyclopedia of Ultrasound in Obstetric and Gynecology, www.isuog.org, March 2024. 


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