Table 1.

Acquisition of US planes, and US measurements performed offline, according to previous literature

Labour mechanism feature	Acquisition plane	US measurements
Fetal head position (FHPo) in the first stage of labour	Transabdominal suprapubic transverse plane	Occiput position (figure 2) Both clinical and US findings of the FHPo are recorded on a data sheet depicting a circle, like a clock, divided into 24 sections, each of 15o, and the position of the occiput is assigned as anterior (OA), posterior (OP), left anterior (LOA), right anterior (ROA), left posterior (LOP), right posterior (ROP), right transverse, left transverse.5 42 The position of the occiput is determined based on the identification of the midline, thalami, choroid plexus, cerebellum, orbits or occiput.
FHPo in the second stage of labour, evaluation of head rotation	Transperineal infrapubic transverse plane (figure 3), with visualisation of the cerebral midline	Occiput position Midline angle (MLA) (figure 3) The position of the occiput (OA, OP, LOA, ROA, LOP, ROP) is determined in a similar fashion transperineally in the 2nd stage of labour. MLA is calculated25 based on the visualisation of the cerebral midline in relation to the anteroposterior axis of the maternal pelvis—rotation angle or MLA.
Fetal head descent/progression	Transperineal translabial sagittal plane	Progression angle26 27 (figure 4), as the angle between the longitudinal axis of the pubic symphysis and the line running from the anterior edge of the pubic symphysis tangentially to the leading edge of the fetal skull. Progression distance22 (figure 4), as the minimal distance between a vertical line from inferior apex of the symphysis (infrapubic line) and the leading edge of the fetal skull. Head direction angle23 24 (figure 4), as the direction of the line perpendicular to the widest diameter of the fetal head, with respect to the infrapubic line.
	Transperineal translabial transverse plane, at the level of the ischial tuberosity, applying firm pressure without creating discomfort, and the transducer moved and angled until the shortest distance to the fetal skull is visualised	Head to perineum distance,43 as the shortest distance from the outer bony limit of the fetal skull to the skin surface of the perineum (figure 4).
Caput and moulding	Transperineal sagittal and transverse plane	Caput (figure 5) is measured as the maximum distance between the leading part of the skull and the fetal skin in the sagittal or transverse planes. Moulding (figure 5) is diagnosed when the skull bones were seen overlapping in the sagittal or transverse planes.

US, ultrasound.