Mechanisms of Labour (Cardinal Movements)

• Pelvic dimensions and configuration
• Fetal size
• Strength of uterine contractions

The 7 Cardinal Movements

1. Engagement

• The fetus is considered engaged when the presenting part is at station 0 (level of the ischial spines)
• In primiparous patients, engagement typically occurs in the last 2 weeks of pregnancy
• In multiparous patients, it may only occur at the onset of labour
• The head usually engages in the transverse or oblique diameter of the pelvic inlet

2. Flexion

• The fetal chin tucks onto the chest, converting the presenting diameter from the occipitofrontal (~11 cm) to the smaller suboccipitobregmatic diameter (~9.5 cm)
• This is mostly a passive movement driven by the resistance of the pelvic floor and walls
• Flexion is necessary for both engagement and descent through the narrowest pelvic dimensions

3. Descent

• The downward passage of the presenting part through the pelvis
• Progressive but not necessarily continuous - it accelerates during the second stage
• Driven by:
  • Uterine contractions
  • Abdominal wall contractions (maternal pushing effort)
  • Straightening and extension of the fetal body

4. Internal Rotation

• As the head descends to the level of the ischial spines, it rotates so the occiput moves from a transverse position anteriorly toward the symphysis pubis (occiput anterior - most common)
• Less commonly, the occiput rotates posteriorly toward the sacral hollow (occiput posterior - persistent OP)
• This rotation brings the long axis of the fetal head into alignment with the anteroposterior (AP) diameter of the pelvic outlet
• Internal rotation is necessary to traverse the ischial spines

5. Extension

• Once the head reaches the vaginal introitus, the occiput comes to lie beneath the inferior margin of the pubic symphysis
• The head pivots around the symphysis - it is born by extension (not further flexion)
• The sequence of delivery: occiput → bregma → forehead → nose → mouth → chin
• After delivery, the head drops downward so the chin lies over the maternal anal region

6. External Rotation (Restitution)

• After the head is delivered, it rotates back to the transverse position it occupied at engagement - aligning with the fetal torso
• This is entirely passive and reflects the natural unwinding of the neck
• It signals that the fetal shoulders are now rotating internally to align with the AP diameter of the pelvic outlet

7. Expulsion

• The shoulders descend through the pelvis in the same path traced by the head
• The anterior shoulder is delivered beneath the symphysis pubis first
• The posterior shoulder is then delivered across the perineum
• The rest of the fetal body follows by lateral flexion

Breech Presentation - Key Differences

• The bitrochanteric diameter is the widest engaging diameter (instead of the BPD)
• The hips usually engage in an oblique diameter of the inlet
• The anterior hip descends faster; internal rotation brings the bitrochanteric diameter into the AP diameter of the outlet
• Lateral flexion delivers the posterior hip first as the anterior hip catches under the symphysis
• Delivery of the aftercoming head (last part to deliver) is the most hazardous step - it requires controlled flexion, descent, and rotation, managed with techniques such as the Mauriceau-Smellie-Veit manoeuvre

Summary Table