MECHANISM OF SECOND STAGE OF LABOR

These are the series of movements that occur on the head in the process of adaptation, during its journey through the pelvis.

Principles of Mechanism:

• Descent takes place throughout labor
• Whichever part leads and first meets the resistance of the pelvic floor will rotate until it comes under the symphysis pubis
• Whatever emerges from the pelvis will pivot around the pubic bone

MAIN MOVEMENTS OF THE FETUS

Includes:

• Engagement
• Descent
• Flexion
• Internal rotation
• Crowning
• Extension of the head
• Restitution
• External rotation
• Birth of shoulder and trunk

Engagement

This is when the bi-parietal diameter of the head passes the pelvic inlet. In nulliparous women; occurs before the onset of active labor because the firmer abdominal muscles directs the presenting part into the pelvis.

In multiparous women; the head often remains freely movable above the pelvic brim till active labor is established due to the relaxed musculature.

Descent

Occurs continuously throughout labor till the fetal head is born.

Throughout the first SOL the contraction and retraction of the uterine muscles allow less room in the uterus, exerting pressure on the fetus to descend further. Descent occurs due to:

• • Uterine contractions
  • Amniotic fluid pressure
  • Abdominal muscle contraction

Flexion

This is very essential for the descent since it reduces the shape and size of the plane of the advancing diameter of the head.

As soon as the head meets resistance from the cervix, pelvic wall or pelvic floor during descent, full flexion is achieved.

Internal Rotation

Occiput rotates anteriorly and the fetal head assumes an oblique orientation. Without it, there will be no further descent.

Torsion of the neck is an inevitable phenomenon during internal rotation of the head. There is no movement of the shoulders from the oblique diameter as the neck sustains a torsion of only 1/8th of a circle. The AP diameter of the head now lies in the widest AP diameter of the pelvic outlet.

Crowning

After internal rotation of the head, further descent occurs until the sub-occiput lies underneath the pubic arch.

At this stage, the maximum diameter of the head (bi-parietal) stretches the vulvar outlet without any recession of the head even after the contraction is over.

Extension of the Head

Once crowning has occurred the fetal head can extend, pivot on the sub-occipital region around the pubic bone.

This releases the sinciput, face, and chin, which sweep the perineum, and then they are born by a movement of extension.

Restitution

The twist in the neck of the fetus which resulted from internal rotation is now corrected by a slight untwisting movement.

The occiput moves 1/8 of a circle towards the side from which it started.

External Rotation

Movement of rotation of the head visible externally due to internal rotation of the shoulders.

As the anterior shoulder rotates towards the symphysis pubis from the oblique diameter, it carries the head in a movement of external rotation through 1/8th of a circle in the same direction as restitution.

Birth of Shoulder and Trunk

After the shoulders are positioned in AP diameter of the outlet, further descent takes place until the anterior shoulder escapes below the symphysis pubis first, by a movement of lateral flexion of the spine, the posterior shoulder sweeps over the perineum. The rest of the trunk is then expelled out by lateral flexion.