This chapter should be cited as follows:
Eggebø TM, Glob. libr. women's med.,
ISSN: 1756-2228; DOI 10.3843/GLOWM.419163

The Continuous Textbook of Women’s Medicine Series – Obstetrics Module

Volume 18

Ultrasound in obstetrics

Volume Editor: Dr Valentina Tsibizova, PREIS International School, Firenze, Italy

Chapter

Ultrasound Evaluation of Assisted Vaginal Delivery

First published: July 2024

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INTRODUCTION

Cesarean section is a lifesaving procedure when medically indicated and should be available for all delivering women.¹ National cesarean section rates of up to 19% have been associated with lower maternal or neonatal mortality,² and this frequency is close to a worldwide estimate of 21% in the period 2010–2018.³ However, the cesarean section rates vary, ranging from 5% in sub-Saharan Africa to 40–50% in other countires.³ Economic, cultural, and organizational factors are important, but also professional practice style and fear of litigation are associated with high cesarean rates.⁴ Unnecessary cesarean sections are associated with increased risk of maternal and neonatal complications.¹ Especially second stage cesarean sections are associated with high risks.⁵ Unnecessary first cesarean sections should be avoided, because of increased risk of uterine rupture and abnormal placentation in subsequent deliveries.¹

Instrumental vaginal delivery is an alternative to late-stage cesarean section. The procedure is often beneficial for the women because it avoids cesarean section and its associated morbidities, and can be performed safely more quickly than a cesarean section.⁶ In the United Kingdom and Scandinavian countries more than 10% of women are delivered with vacuum extraction or forceps.^7,8 However, in low- and mid-resource countries an underuse of these procedures with less than 1% in many institutions has been reported.^9,10 A decreased frequency from 9.0% to 3.3% was also observed in the United States from 1992 to 2013.⁶

The most common indications for an operative vaginal birth are prolonged second stage of labor, suspicion of immediate or potential fetal compromise and shortening of the second stage of labor for maternal benefit.⁶ Prerequisites before performing the procedure are a fully dilated cervix, ruptured membranes, engagement of the fetal head and exact knowledge of fetal position. These prerequisites are included in the guidelines of the American College of Obstetricians and Gynecologists (ACOG).⁶ The green top guideline from the Royal College of Obstetricians and Gynaecologists prescribes that a safe assisted vaginal birth requires a careful assessment of the clinical situation, clear communication with the woman and healthcare personnel, and expertise in the chosen procedure.¹¹ However, the clinical assessments of fetal descent and position are subjective and unreliable.^12,13 Ultrasound assessment of fetal position is more accurate than clinical examination,¹⁴ and therefore, has the potential to improve outcome. However, the benefit of examining fetal position has not yet been proven in randomized studies.¹⁵

The six basic steps in general ultrasound examinations are to examine the number of fetuses, check fetal heart rate and placental location, estimate the amount of amniotic fluid, assess fetal lie and presentation, and do basic measurements.¹⁶ These six steps can also be examined during active labor, however, the number of fetuses and placental location are usually known before labor onset. Confirm vitality before doing a cesarean section. Cesarean section should usually be avoided on dead fetuses. This is only necessary in women with placenta previa, uterine rupture or transverse lie.

The orientation of the fetus in uterus can be described as fetal lie, position and attitude. The fetal lie can be longitudinal, transverse or oblique. Position means the orientation of the fetal back and fetal head related to the mother, and presentation describes the lowermost part of the fetus in the birth canal. Presentation is usually breech or cephalic, but in cases with transverse lie the presenting part can be the shoulder or the arm. Presentation of the head can be subdivided into occiput, sinciput, brow or face presentation. An exact knowledge about fetal orientation and descent in the birth canal is essential before an operative vaginal delivery. A vacuum cup should be placed over the flexion point and exact knowledge about position is a prerequisite before applying forceps. The direction of traction differs in occiput anterior and occiput posterior positions. In anterior position the fetus can directly follow the curve of the birth canal and extend the neck as soon as the head has passed the symphyses pubis. In occiput posterior position the fetus descends deeper in the birth canal before being directed upwards and the traction should be horizontal to a lower level before directed upwards.¹⁷ Video 1 shows traction direction in occiput anterior position and Video 2 in occiput posterior position.

ULTRASOUND ASSESSMENT OF FETAL ORIENTATION

Start with a transabdominal scan. The ultrasound examination should be done in a structured way, starting with transverse scans followed with a sagittal scan in the midline and both sides (Videoclip 3).

Fetal lie is essential to know, consider that 20% of breech presentations are undiagnosed at admission in labor wards. The fetal lie can be easily assessed by a transabdominal scan.

Fetal position has traditionally been categorized into eight equal sectors that are also used in some ultrasound studies, however, a simpler method only categorizing into anterior, transverse and posterior positions was suggested by Akmal et al. (Figure 2).¹⁸ They described position like a clock face: positions ≥10.00 and ≤02.00 as occiput anterior (OA), positions ≥02.30 and ≤03.30 as left occiput transverse (LOT), positions ≥04.00 and ≤08.00 as occiput posterior (OP) and positions ≥08.30 and ≤09.30 as right occiput transverse (ROT).¹⁸ This method is recommended by ISUOG.¹⁹

Fetal position can usually be determined by a transverse transabdominal scan. Important landmarks are the fetal eyes, the thalami, the cerebellum and the choroid plexus which are diverging towards the occiput (Figures 3–5 and Videoclips 4–6).²⁰ A longitudinal transabdominal scan is helpful for non-experienced sonographers because the spine can easily be seen (Figure 6 and Videoclip 7), but the midline structures in the fetal head should be obtained in transverse scans for exact determination of position (Figures 3 and 5). At low stations a transperineal scan may be added (Figure 5). Occiput anterior position is associated with an easy operative vaginal delivery; however, operative vaginal procedures can also be done from transverse and posterior positions.²¹ In these situations, trained birth attendants should be available, and immediately conversion to a cesarean section should be possible if the vaginal procedure fails. One study found that 60% of occiput posterior positions rotated to occiput anterior during a vacuum extraction.²¹

Fetal attitude can in occiput anterior and transverse positions be examined as the angle between the cervical spine and the occipital bone (occiput spine angle).²³ In early labor the fetus flexes the neck (Figure 6).

Late in labor the fetus extends the neck. The occiput spine angle is difficult to measure at low stations due to shadowing from the symphysis pubis, but the extension can be seen as head is directed upwards (Figure 7).

Head direction is downwards in early labor, changing to horizontal and upward direction during labor progression. The change in head direction is caused by the curved birth canal and by the change in attitude. The head direction should be observed before an operative vaginal delivery, and upward direction is associated with an easy procedure.²⁴

ULTRASOUND ASSESSMENT OF FETAL DESCENT

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 with a grading system from −5 to +5 cm (or −3 to +3 in some countries). Engagement is based on a transabdominal examination of the proximal part of the fetal head above the pelvic inlet. The “fifths” method is used for clinical palpation of the fetal head above the pelvic inlet, and engagement occurs when only two-fifths of the head or less is palpable above the brim.²⁵ Engagement is considered as a prerequisite for an operative vaginal delivery.¹¹ The fifths method is inexact and poorly reproducible, but it is still used as the only method to assess descent in the World Health Organization labor care guide from 2020.²⁶

Fetal level in the birth canal can be examined with a transperineal ultrasound examination. The transducer should be placed between the labia as shown in videoclip 8. The frontal plane will be visualized in a transverse scan (videoclip 9), and the sagittal plane in a longitudinal scan (videoclip 10).

Fetal station can be expressed as angle of progression (AoP) or as head-perineum distance (HPD).^27,28 AoP is measured in a sagittal scan as the angle between a line through the long axis of the symphysis pubis and a second line from the inferior end of the symphysis pubis tangentially to the contour of the fetal skull. (Figure 8). 120 degrees corresponds to clinical station 0 and 140 degrees corresponds to clinical station +2.

HPD is measured as the shortest distance from the fetal skull to perineum, representing the remaining part of the birth canal for the fetus to pass. The ultrasound probe should be placed transversely between labia majora, and the soft tissue should be compressed completely against the pubic bone and the transducer angled until the skull contour is as clear as possible, indicating that the ultrasound beam is perpendicular to the fetal skull (Figures 5 and 9). 35 mm corresponds to clinical station 0 and 25 mm corresponds to clinical station +2.

Both methods can predict the probability of a successful operative vaginal delivery.^31,27,32 An AoP >120 degrees was associated with an easy vacuum delivery in fetuses in OA position in the original studies,^27,32 however, another study including fetuses in all position recommended a cut-off level of 145 degrees.³³ HPD ≤35 mm has been recommended as cut-off level for a vacuum attempt,³¹ but the failure rate was higher in posterior and transverse positions compared with anterior positions.²¹

ULTRASOUND ASSESSMENT IN THE THIRD STAGE OF LABOR

Uterine atony and retained placental tissue are the most common causes of postpartum hemorrhage. The uterus can be examined with a transabdominal scan after delivery. Retained placental tissue can be seen as echogenic tissue in the uterus (Figure 10). A small amount of blood is often seen in the uterus after delivery as a normal finding. However, a large amount of blood within the uterus, the cervix (Figure 11) or in the vagina should be manually removed.

CONCLUSIONS

Ultrasound has the potential to reassure clinicians that an operative vaginal delivery is a safe procedure in fetuses at low stations. When examined between contractions, favorable prognostic factors in anterior positions are head direction upwards, AoP >120 degrees and HPD <35 mm (corresponding to clinical station 0). More caution should be shown in transverse and posterior positions. Favorable prognostic signs in these situations are head direction upwards, AoP >140 degrees and HPD <25 mm (corresponding to clinical station +2). Movement of the fetal head during contractions and with pushing is also a favorable factor.

PRACTICE RECOMMENDATIONS

ACKNOWLEDGMENT

The author would like to thank Morten Dreier for creating Videos 1, 2, 3 and 8, and to thank Johanne Kolvik Iversen for acquiring ultrasound images in Figures 7 and 8.

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