Mechanism of Labour
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Direct answer
Seven cardinal movements carry a well-flexed occipito-anterior vertex through the pelvis: engagement, descent, flexion, internal rotation, crowning, extension (with restitution and external rotation), and expulsion. The fully flexed head enters the brim in an oblique diameter with the suboccipitobregmatic diameter of 9.5 cm, descends with each contraction, and rotates at the ischial spines so the occiput turns 45 degrees anteriorly. Birth occurs not by flexion but by extension around the subpubic arch, after which restitution and external rotation align the head with the shoulders. A mechanism gone wrong is simply the anatomy of obstructed labour.
What you must remember
- Engagement timing: in a nullipara the head normally engages by 38 weeks; a floating head at term in a primigravida suggests deflexion, occipitoposterior position, pelvic contraction, placenta praevia or tumour; in multiparas engagement may wait for labour.
- Engaging diameters: fully flexed vertex — suboccipitobregmatic 9.5 cm; deflexed (military attitude) — occipitofrontal 11.5 cm; brow — mentovertical 13.5 cm (undeliverable at term); face — submentobregmatic 9.5 cm again.
- Internal rotation: occurs at the pelvic floor, at the level of the ischial spines, produced by the sloping gutter of levator ani; the occiput, being the lower and leading pole, rotates 45 degrees (from occipito-transverse) or 90 degrees (from occipitoposterior) to become occipito-anterior.
- Asynclitism: anterior parietal presenting (Naegele) is more favourable; posterior parietal (Litzmann) traverses a longer arc and carries a worse prognosis.
- Crowning and birth: crowning means the widest biparietal diameter has passed the vulval outlet and the head no longer recedes between contractions; the head is then born by extension — chin, mouth, nose, forehead, and occiput appearing successively over the perineum.
- Restitution versus external rotation: restitution is passive untwisting of the head (45 degrees) to align with the shoulders; external rotation is a further 45-degree turn transmitted from inside as the anterior shoulder rotates internally.
- Shoulders and trunk: the anterior shoulder is born first under the symphysis, then the posterior shoulder over the perineum; lateral flexion of the trunk delivers the body.
Walking an LOA vertex down, step by step
Take a left occipito-anterior vertex at term. The head engages with the sagittal suture in the right oblique diameter of the brim, occiput to the mother's left. Descent continues along the pelvic curve while flexion deepens, because the resistance of cervix and floor converts the applied force into chin-to-chest apposition — every millimetre of flexion substitutes the round 9.5 cm suboccipitobregmatic for the longer occipitofrontal plane. When the leading occiput reaches the levator hiatus, the inclined planes of the pelvic floor guide it forward: internal rotation is a passive turning imposed by the gutter of the outlet, not a muscular act of the fetus. With the occiput under the symphysis, descent proceeds until the biparietal eminences crown; the subocciput then acts as the fulcrum against the pubic arch, and the head extends and escapes — nape, brow, face in turn. Restitution turns the face to the mother's left thigh and external rotation to the right, mirroring the shoulders inside; the anterior shoulder delivers under the symphysis, the posterior over the perineum, and the trunk follows by lateral flexion.
How Indian wards and vivas frame it
District hospitals still receive the failure modes of this mechanism — the unbooked multigravida with a neglected transverse lie or an impacted occipitoposterior, a moulded caput at the outlet while the head never rotated, and Bandl's ring rising above the umbilicus; recognising that picture before uterine rupture is why the mechanism must be understood, not recited. Indian examiners fixate on three facts: the muscle responsible for internal rotation (levator ani), the diameter that engages with full flexion (suboccipitobregmatic, 9.5 cm), and why a mentoposterior face cannot deliver (the short neck and thorax jam against the perineum — only mento-anterior, using the submentobregmatic 9.5 cm, delivers vaginally). A favourite trap separates restitution, the passive untwisting of the delivered head, from external rotation, which belongs to the shoulders.
Frequently asked questions
Which diameter engages when the vertex is well flexed?
The suboccipitobregmatic diameter of 9.5 cm, engaging in the right oblique diameter of the brim in a left occipito-anterior position.
At what level and by what muscle does internal rotation occur?
At the pelvic floor at the level of the ischial spines; the levator ani gutter rotates the occiput anteriorly by 45 to 90 degrees.
Why is a mentoposterior face undeliverable at term?
The chin lies behind and the thorax presses against the perineum, so no mechanism brings the submentobregmatic diameter into the outlet axis; it must rotate to mento-anterior or end in caesarean.
What differentiates restitution from external rotation?
Restitution is passive untwisting of the delivered head to realign with the shoulders; external rotation is transmitted from the shoulders' internal rotation inside.
What does severe moulding with a large caput at the outlet signify?
That the head has been forced down without completing the mechanism — typically a persistent occipitoposterior or deep transverse arrest — and that the labour is obstructed, not progressing.