The labour ward is a demanding environment. Clinical decisions are made under time pressure, often with imperfect information. Digital vaginal examination – our traditional tool for assessing fetal head position and station – is unreliable, with error rates of 20–70% even among experienced operators.¹ This inaccuracy is most pronounced precisely when it matters most: in malposition, asynclitism, and obstructed labour. Intrapartum ultrasound offers an adjunct to assist assessment, and the techniques required need not be complex.

This article distils the most accessible techniques for use in labour that are learnable without specialist training.

Why Bother? The Case for Labour Ward Ultrasound

Current evidence identifies two situations where ultrasound adds most value:²

• Before operative vaginal delivery
• Suspected delay or arrest of labour

Error in fetal head position assessment, moulding, and caput succedaneum affect digital vaginal assessment and this translates to operative risk. Incorrect cup or forceps placement increases the likelihood of fetal injury and incidence of a failed instrumental birth followed by caesarean birth.² A randomised controlled trial demonstrated that ultrasound assessment prior to instrumental birth reduced position misdiagnosis from 20.2% to 1.6% compared with digital examination alone.⁴

Beyond operative birth, ultrasound provides objective information about labour progress. In prolonged first or second stage, the angle of progression (AOP) correlates with the likelihood of a caesarean birth, which may change both counselling and clinical management.² Women also consistently prefer ultrasound to digital examination in labour.³

What Equipment Do You Need?

Any standard convex abdominal probe (< 4 MHz) with a wide sector width is all that is required for the techniques described here.²

Getting the Most from Your Machine

Published guidelines say little about machine optimisation for intrapartum scanning beyond probe selection. The following practical tips draw on general principles.

Preset

Start with the abdominal obstetric preset if available – this configures frequency, depth, and processing defaults appropriate for late pregnancy. If this is not available, a general abdominal preset is a reasonable alternative.

Depth

For transabdominal head position assessment, 10–14cm is typical. For transperineal AOP measurement – where the pubic symphysis and leading edge of the fetal skull are the key landmarks, 6–8cm is usually sufficient. The most common trans-perineal error is insufficient depth, which cuts off the inferior margin of the symphysis pubis and makes AOP measurement impossible; if the symphysis pubis appears incomplete at the top of the image, increase depth before attempting a measurement.

Tissue harmonic imaging (THI)

THI improves contrast resolution and reduces near-field artefact and is the default on most modern machines. It is generally beneficial for transabdominal imaging, particularly in women in larger bodies. However, THI can reduce the signal at very shallow depths. For transperineal imaging, where the probe is close to the symphysis, switching THI off can improve the image.

Four Techniques Any Clinician Can Learn

These techniques require only basic point-of-care ultrasound competency. Research demonstrates that even the angle of progression can be learned reliably regardless of prior ultrasound experience.⁷

1. Transabdominal Fetal Head Position

This is the most important and accessible technique. Place the probe transversely on the abdomen at the level of the fetal chest to identify the spine. Then slide downwards to the suprapubic region to visualise the fetal head in axial section.²

Three landmarks guide position classification:

• Two anechoic circles representing the two fetal orbits visible anteriorly – occiput posterior (OP) (Figure 1).
• Echogenic line through the centre of the axial head corresponds to the midline falx echo – occiput transverse (OT).
• Echogenic lines representing the posterior vertebral elements connected to the head represent the cervical spine and occiput anteriorly – occiput anterior (OA) (Figure 2).

At low fetal head stations, transabdominal visualisation of the midline structures may be difficult. In these cases, a transperineal approach in the axial plane allows identification of the diverging choroid plexuses pointing towards the occiput.²

Fig. 1. Occiput posterior (OP) presentation on ultrasound. Image courtesy of Dr Karen Mizia, Ultrasound Care Australia.

Fig. 2. Occiput anterior (OA) presentation on ultrasound. Image courtesy of Dr Karen Mizia, Ultrasound Care Australia.

2. Fetal Spine Localisation

If the fetal head is difficult to image, the spine position provides a reliable proxy for occiput position.

3. Angle of Progression (AOP)

This is the most studied transperineal parameter and the most clinically useful measure of fetal head station. In the semi-recumbent position and an empty bladder, the probe is placed in the mid sagittal plane on the perineum between the two labia majora or more caudally at the level of the posterior fourchette. The pubic symphysis, seen as an oblong echogenic structure, forms the landmark for measurement with the fetal skull. The AOP is the angle between the long axis of the pubic symphysis and a line drawn from its lower edge to the leading edge of the fetal skull (Figures 3-4).²

Fig. 3. Illustration of angle of progression (AOP). Image with permission from GE Australia.

Although the ischial spines are not seen in this view, there is a fixed relationship between the inferior edge of the symphysis pubis and the ischial spines which allows extrapolation of the fetal head station, as the ischial spines lie parallel and 3cm below the inferior pubic symphysis.

Clinically meaningful thresholds:

• AOP ≥ 120°: predicts successful vacuum extraction in approximately 90% of occiput-anterior cases.²
• AOP > 110° in prolonged first stage: caesarean birth rate approximately 12%.²
• AOP < 100° in prolonged first stage: caesarean birth rate approximately 62%.²
• AOP of 116° corresponds to station 0.

Fig. 4. Ultrasound image angle of progression (AOP). Image with permission from GE Australia.

4. Head Direction

In the midsagittal transperineal view, the orientation of the longest axis of the fetal skull relative to the pubic symphysis is an indirect marker of head station and correlated with descent: “head down”, where the direction of the head is pointing away from the pubic symphysis, suggests a high station. A “horizontal” position where the head lies parallel to the pubic symphysis suggests mid-cavity, and “head up”, with the head pointing towards the symphysis and probe a low station. The “head up” sign prior to instrumental birth correlates with successful and relatively straightforward vacuum extraction.²

Infection Prevention: What Obstetricians Need to Know

Hospital outbreaks have been traced to contaminated multi-use gel bottles.⁶ Multi-use bottles must never be refilled. Single-use sachets are preferred where feasible.

Ultrasound probes can transmit infection between patients if not decontaminated correctly. The principle is simple: the more invasive the probe use, the more rigorous the cleaning required.⁶ For the techniques described in this article, there are two practical scenarios:

Transabdominal examinations, which touch only intact skin require the same level of disinfection as a routine obstetric scan. Clean the probe promptly after use before gel dries, then wipe with an approved disinfectant compatible with your machine.

Transperineal examinations have the potential to contact mucous membranes and blood. They require more rigorous decontamination. A sterile single-use cover must be applied before use, and the probe requires high-level disinfection (HLD) afterwards – regardless of whether the cover appeared intact. Single-use sterile gel must be used throughout.⁶

How to Build Competency

Studies suggest that basic intrapartum ultrasound skills can be acquired with as little as 20 supervised examinations.⁷ The International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) 2018 guidelines include a freely available teaching video designed specifically for non-expert clinicians, providing an accessible starting point for self-directed learning before supervised practice.²

Importantly, ultrasound is not a substitute for clinical examination or as routine assessment in uncomplicated labour.² One randomised trial found routine use in low-risk patients was associated with higher caesarean rates.⁸ The tool is most valuable when clinical uncertainty is highest.

Conclusion

Intrapartum ultrasound is not the exclusive domain of subspecialists. The core techniques – transabdominal head position, fetal spine localisation, AOP measurement, and head direction – are learnable, reproducible, and useful. In the context of operative obstetrics, they reduce positional misdiagnosis, improve instrument placement, and support better-informed consent and clinical decision-making.