Mechanism of Labour (Cardinal Movements)

1. Engagement

• The biparietal diameter (BPD) of the fetal head passes through the pelvic inlet (plane of the pelvic brim).
• In primigravidas, engagement typically occurs in the last few weeks of pregnancy; in multigravidas, it often happens only during labour.
• The head usually engages in the transverse or slightly oblique diameter of the pelvis.
• Clinically assessed by the amount of fetal head palpable above the pelvic brim (measured in fifths).

2. Descent

• Progressive downward movement of the fetal head through the pelvic canal.
• Occurs throughout labour, but is most rapid during the second stage.
• Driven by: uterine contractions, bearing-down efforts, and gravity.
• Descent is essential for all other movements to occur.

3. Flexion

• As the head descends and meets resistance from the pelvic floor, the chin is brought closer to the fetal chest.
• Flexion converts the presenting diameter from the occipitofrontal (11.5 cm) to the smaller suboccipitobregmatic diameter (9.5 cm), making passage easier.
• This is a passive movement.

4. Internal Rotation

• The fetal head rotates so that the occiput moves from its original transverse or oblique position anteriorly toward the symphysis pubis (in OA position).
• This aligns the long axis of the head with the anteroposterior diameter of the pelvic outlet.
• Occurs as the head reaches the level of the ischial spines (level of the pelvic floor / mid-cavity).
• Caused by the shape of the levator ani muscle and the gutter-shaped pelvic floor.
• In occipitoposterior (OP) positions, internal rotation may be incomplete or reversed, leading to prolonged labour.

5. Extension (Crowning and Delivery of the Head)

• As the occiput reaches the pubic arch, the head pivots around the symphysis pubis.
• The presenting part extends rather than flexes further, allowing the head to be born by extension.
• Sequence of delivery: occiput → bregma → forehead → nose → mouth → chin.
• This occurs at the vulval outlet.

6. Restitution (External Rotation - Part 1)

• Immediately after the head is delivered, it rotates back (untwists) to resume its natural alignment with the fetal shoulders.
• The occiput rotates 45° toward the mother's thigh on the same side as the original position.
• This movement aligns the head back with the shoulders.

7. External Rotation (Shoulder Rotation)

• The shoulders now undergo internal rotation, rotating from the oblique to the anteroposterior diameter of the pelvic outlet.
• This causes the head to rotate a further 45° so the occiput faces one of the mother's thighs directly.
• The anterior shoulder is delivered first under the pubic arch, then the posterior shoulder sweeps over the perineum.

8. Expulsion

• After delivery of the shoulders, the rest of the fetal body (trunk and legs) is expelled rapidly by lateral flexion.

Summary Table

Important Landmarks

• Pelvic inlet: widest in the transverse diameter (13 cm) - head enters transversely
• Mid-cavity (at ischial spines): narrowest part - internal rotation happens here
• Pelvic outlet: widest in the anteroposterior diameter (13.5 cm) - head exits AP

Notes on Abnormal Mechanisms

• Occipitoposterior position: internal rotation is to the posterior (long rotation of 135°), causing prolonged labour and risk of instrumental delivery
• Face presentation: mentoanterior positions can deliver vaginally; mentoposterior cannot
• Brow presentation: usually requires cesarean delivery (largest presenting diameter)
• Breech presentation: entirely different mechanism (extended breech, flexed breech, footling)