Labour is the physiological process by which the products of conception are expelled from the uterus through the vagina to the outside world. For labour to be classified as normal or eutocia, specific criteria must be met. It must occur at term, starting spontaneously without artificial induction, and present with a cephalic presentation where the baby's head leads the way. The process must progress through regular uterine contractions that cause progressive effacement and dilatation of the cervix. Crucially, normal labour terminates with the spontaneous vaginal delivery of a live infant without requiring complex instrumental assistance or surgical intervention, and it must conclude without causing undue complications to either the mother or the newborn.
The mechanism of labour describes the passive movements the fetus makes while navigating the irregularly shaped maternal pelvis. These movements are driven primarily by forceful, rhythmic uterine contractions coupled with maternal pushing efforts. Four main factors determine the success of this journey: the power of the uterine contractions, the passenger (size, lie, and presentation of the fetus), the passage (anatomy of the maternal pelvis), and the psyche of the mother. As the fetus descends, it does not simply slide down in a straight line. The pelvic inlet, cavity, and outlet force the baby to make specific positional adjustments to fit through the smallest diameters of the maternal bony pelvis.
As the descending fetal head meets resistance from the cervix, pelvic walls, or pelvic floor, involuntary flexion occurs. The baby's chin is pushed firmly into contact with its chest. This movement changes the diameter of the presenting head from the wide occipitofrontal measurement to the much smaller, more favorable suboccipitobregmatic measurement. Effective flexion allows the smallest possible circumference of the skull to present to the maternal pelvis, making the journey much easier and more efficient.
Internal rotation is the movement where the fetal head turns inside the pelvis, usually bringing the occiput from its original position to lie anteriorly beneath the pubic symphysis. As the head descends past the pelvic floor muscles, which are shaped like a downward-sloping slide, the lowest part of the head turns forward to take the path of least resistance. This vital maneuver aligns the long anteroposterior diameter of the fetal head with the wide anteroposterior diameter of the pelvic outlet, preparing the baby to emerge under the pubic arch.
Once internal rotation is complete and the head reaches the perineum, extension occurs. The base of the occiput pivots directly against the inferior margin of the maternal pubic symphysis, acting as a fulcrum. As uterine contractions push the fetus downward and the perineal tissues offer backward resistance, the head is forced to sweep upward and out of the vagina. The vertex, brow, face, and chin successively pass over the stretched perineum in an arcuate motion until the head is fully born.
The final stage of the mechanism of labour is the delivery of the trunk and limbs. Once external rotation is complete, the anterior shoulder appears under the pubic symphysis and is delivered first by lateral flexion of the body upward over the perineum. Next, the posterior shoulder is delivered as the body flexes downward. After both shoulders are free, the rest of the baby's body—the trunk, torso, and lower limbs—follows rapidly and smoothly along the curve of the birth canal due to residual expulsive forces. The complete emergence of the infant marks the successful conclusion of the second stage of labour.