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Mechanism of labor

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labor AND physiology

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mechanism of labour cardinal movements fetal head WHO obstetric labour physiology

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https://www.ncbi.nlm.nih.gov/books/NBK544290

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Mechanism of labor

The mechanism of labor is the series of positional adaptations made by the fetus, especially the head, to pass through the maternal pelvis during a normal vaginal delivery. It is usually described for a singleton, term fetus in vertex presentation with occiput anterior position.
It depends on the interaction of:
  • Powers: uterine contractions and maternal bearing-down efforts
  • Passenger: fetal size, presentation, position, and attitude
  • Passage: maternal bony pelvis and soft tissues
The fetal head follows the path of least resistance by presenting its smallest possible diameter at each level of the pelvis. NCBI overview of normal labor describes this alignment as the basis for the cardinal movements.

Cardinal movements of labor

  1. Engagement
    The biparietal diameter of the fetal head passes through the pelvic inlet.
    • The head is considered engaged when its widest transverse diameter has crossed the inlet, conventionally at station 0.
  2. Descent
    The presenting part moves progressively downward through the pelvis.
    • It is produced mainly by uterine contractions, later aided by maternal pushing in the second stage.
    • Descent occurs throughout labor but becomes most marked in the second stage.
  3. Flexion
    As the descending fetal head meets resistance from the cervix, pelvic walls, and pelvic floor, the chin comes closer to the chest.
    • This changes the presenting head diameter from the larger occipitofrontal diameter to the smaller suboccipitobregmatic diameter (about 9.5 cm), helping passage through the pelvis.
  4. Internal rotation
    The occiput rotates forward, usually through about one-eighth of a circle, from a transverse or oblique position toward the anterior position.
    • Thus, the sagittal suture comes to lie in the anteroposterior diameter of the pelvic outlet.
    • Most commonly, the occiput comes under the maternal symphysis pubis, producing an occipitoanterior delivery.
  5. Extension
    When the occiput is fixed beneath the symphysis pubis, the head extends to negotiate the curve of the birth canal.
    The head is born in sequence:
    • Occiput
    • Brow
    • Face
    • Chin
  6. Restitution
    After delivery of the head, it rotates about one-eighth of a circle to correct the twist produced in the neck during internal rotation.
    • The head realigns with the shoulders.
  7. External rotation
    The shoulders rotate internally into the anteroposterior diameter of the pelvic outlet. Simultaneously, the delivered head rotates externally in the same direction.
  8. Expulsion
    The anterior shoulder is delivered under the symphysis pubis, followed by the posterior shoulder over the perineum. The trunk and lower limbs then follow rapidly.

Sequence to remember

Engagement → Descent → Flexion → Internal rotation → Extension → Restitution → External rotation → Expulsion
A useful mnemonic: “Every Darn Fool In Egypt Rides Every Elephant.”
Some texts list descent before flexion, while others describe flexion before descent because these movements overlap in real labor.
The standard vertex mechanism is summarized in Roberts and Hedges’ Clinical Procedures in Emergency, p. 1351: engagement, flexion, descent, internal rotation, extension, restitution/external rotation, and expulsion. Tintinalli’s Emergency Medicine likewise lists engagement, flexion, descent, internal rotation, extension, and external rotation as the core cardinal movements.
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