Study Notes on Mechanisms of Labor and Birth

Mechanisms of Labor and Birth

Overview of Key Principles

  • There are three main principles in the mechanisms of labor and birth:
    • Descent and Flexion: Occur continuously throughout labor.
    • Resistance and Rotation: The first presenting part encountering pelvic floor resistance will rotate and rest under the symphysis pubis.
    • Birth Sequence: The part under the symphysis pubis is the first to be born.

Mnemonic for Mechanisms of Labor

  • A helpful mnemonic to remember the steps in the mechanisms of labor:
    • D: Descent of the fetal head
    • F: Flexion of the fetal head
    • I: Internal rotation of the fetal head
    • C: Crowning
    • E: Extension
    • R: Restitution
    • I: Internal rotation of the shoulders
    • E: External rotation of the head (occurs simultaneously with internal rotation of the shoulders)
    • L: Lateral flexion for the birth of the body, arms, and legs of the baby

Step-by-Step Mechanisms of Labor

1. Descent
  • Descent occurs throughout labor, beginning with partial descent.
  • Increased contractions push the fetus down into the pelvis, leading to further descent.
  • Flexion occurs in response to the resistance created from increased pressure due to contractions.
2. Flexion
  • The baby's head flexes in response to the pressures faced in the pelvic cavity.
3. Internal Rotation
  • The orientation of the body:
    • The fetal head is positioned in the right oblique diameter of the pelvis.
    • The fetal shoulders are in the left oblique diameter of the pelvis.
  • The posterior fontanelle points to the left ileopectineal eminence, indicating a left occipital anterior mechanism of labor.
  • As labor progresses, the head descends further, leading to increased flexion where the occiput (posterior fontanelle) is lower than the anterior fontanelle.
  • Upon encountering resistance from the pelvic floor, the head rotates 45 degrees anteriorly and rests under the symphysis pubis.
4. Crowning
  • Crown occurs when:
    • The occiput comes down and causes the perineum to bulge.
    • The head does not recede between contractions.
  • At this stage, the occiput is now born from beneath the symphysis pubis, consistent with the principle that the part under the symphysis pubis will be delivered first.
5. Extension
  • Extension occurs wherein:
    • The sinciput, face, and chin of the baby sweep over the perineum, completing the delivery of the head.
6. Restitution
  • During restitution, the head rotates back 45 degrees to its original position, returning to left occipital anterior, and re-aligns back into the right oblique diameter of the pelvis, undoing the earlier twist in the neck.
7. Internal Rotation of Shoulders
  • The internal rotation of the shoulders isn't externally visible; however:
    • The shoulders remaining in the left oblique diameter encounter resistance at the pelvic floor, rotating 45 degrees anteriorly to rest under the symphysis pubis.
8. External Rotation of Head
  • The head undergoes an additional rotation of 45 degrees to the left, aligning with the transverse position.
9. Lateral Flexion
  • During this step, the anterior shoulder is born first, followed by:
    • An upward lateral flexion for the posterior shoulder's delivery.
  • Finally, the remainder of the body is delivered by lateral flexion over the mother's abdomen.

Summary

  • The mechanics of labor involve a precise sequence of movements (descent, flexion, rotation, crowning, extension, restitution, and shoulder rotations) that work together to facilitate the delivery of the baby through the birth canal, fully adhering to the principles of descent and rotation as related to pelvic geometry and resistance.