The mechanism of labor refers to the series of adaptive movements made passively by the presenting part of the fetus as it navigates through the birth canal, aligning itself with the varying shapes of the planes of the pelvis to pass through with its smallest diameters. Clinically, the left occiput anterior (LOA) position is the most common occiput presentation; thus, the mechanism of labor is typically explained using this presentation as an example. The process includes engagement, descent, flexion, internal rotation, extension, restitution and external rotation, and finally, delivery of the shoulders and the rest of the fetus. Although these movements are described individually, they occur in a continuous sequence during labor.
Figure 1 Diagram of the mechanism of labor in left occiput anterior position
Engagement
Engagement occurs when the biparietal diameter of the fetal head enters the pelvic inlet plane, with the lowest point of the skull approaching or reaching the level of the ischial spines. At this stage, the fetal head typically enters the pelvic inlet in a semi-flexed position, engaging along the occipitofrontal diameter. Since the occipitofrontal diameter is larger than the anteroposterior diameter of the pelvic inlet, the sagittal suture is often aligned with the right oblique diameter of the pelvic inlet. In many primiparous women, engagement takes place 1–2 weeks before the due date, while in multiparous women, it generally occurs after the onset of labor.
Descent
Descent refers to the movement of the fetal head along the pelvic axis. This process continues throughout the entire course of labor and occurs simultaneously with other movements. During uterine contractions, the fetal head progresses downward, and during intervals between contractions, it slightly retracts. This intermittent pressure helps in reducing strain on both the mother and the fetus. Factors contributing to descent include:
- Pressure transmitted to the fetal head via amniotic fluid during uterine contractions,
- Direct compression of the fetal buttocks by the uterine fundus during contractions,
- Elongation of the fetal body, and
- Increased abdominal pressure from maternal muscle contractions.
In primiparous women, descent occurs more slowly due to slower cervical dilation and higher resistance from soft tissues compared to multiparous women. The speed and extent of descent are key clinical indicators of labor progress.
Flexion
As the fetal head continues its descent and reaches the pelvic floor, it encounters resistance from the levator ani muscles. This resistance enhances the flexion of the fetal head, bringing the chin closer to the chest. This movement reduces the diameter of the presenting part from the occipitofrontal diameter to the suboccipitobregmatic diameter, enhancing the ability of the head to descend through the pelvis.
Internal Rotation
When the fetal head reaches the pelvic floor, it encounters resistance, prompting the head to rotate to adapt to the pelvis. In this process, the occiput rotates 45° toward the maternal midline, moving to the area behind the pubic symphysis. This aligns the sagittal suture with the anteroposterior diameter of the mid-pelvis and pelvic outlet. Internal rotation is usually completed by the end of the first stage of labor. In occiput presentation, as the occiput—the lowest part of the fetal head—encounters the resistance of the pelvic floor muscles, the contraction of the levator ani muscles directs the occiput toward the wider and less resistant anterior section.
Extension
After internal rotation, the flexed fetal head reaches the vaginal opening. The combined forces of uterine contractions and abdominal pressure push the fetal head downward, while the levator ani muscles provide supportive resistance. This combined force promotes the progression of the fetal head along the lower section of the pelvic axis. When the suboccipital region of the fetal head reaches the underside of the pubic symphysis, the head pivots at the pubic arch. Gradual extension of the head occurs, allowing the vertex, forehead, nose, mouth, and chin to be delivered sequentially. As the head extends, the shoulders align with the left oblique diameter of the pelvic inlet.
Restitution and External Rotation
After the head is delivered, the shoulders are positioned along the left oblique diameter of the pelvic inlet. To restore the normal anatomical relationship between the head and shoulders, the occiput rotates 45° to the maternal left, a movement known as restitution. The shoulders continue their descent in the pelvic cavity. The anterior shoulder rotates 45° toward the maternal midline, aligning the bisacromial diameter of the fetal shoulders with the anteroposterior diameter of the pelvic outlet. The occiput rotates another 45° externally to the maternal left side, maintaining the perpendicular relationship between the head and shoulders. This movement is referred to as external rotation.
Delivery of the Shoulders and Fetus
Following external rotation, the anterior shoulder emerges under the pubic arch first, followed by the posterior shoulder, which is delivered over the perineum. The rest of the fetal body and lower limbs are subsequently delivered, completing the labor process.