Mechanism -Cardinal Movement of Labor

Subject: Midwifery II (Theory)

Overview

During labor, the fetus changes position, attitude, and presentation. This procedure is required for vaginal delivery to be successful. To adapt to the maternal pelvic dimensions, the fetus must undergo a series of adjustments in the attitude of its presenting part. This is essential for fetal descent via the birth canal. In a vertex presentation, the cardinal labor movements are engagement, descent, flexion, internal rotation, the crowing of the head, extension, restitution of the head, external rotation, and expulsion.

During labor, the fetus undergoes a series of changes in position, attitude, and presentation. This procedure is required for a successful vaginal birth.

Cardinal Movements of Labor

To adapt to the mother's pelvic dimensions, the fetus must experience a series of modifications in its presenting part's attitude. This is necessary for fetal descent along the birth canal. The cardinal labor movements in a vertex presentation are as follows:

  1. Engagement:
    When the biparietal diameter is at or below the level of the pelvic inlet, the vertex is engaged. This equates with an occiput perceptible at 0 stations in clinical practice.
     
  2. Descent:
    This cardinal movement is frequently reported late in the active period of the first stage of labor and occurs concurrently with engagement.

    Factors facilitating descent are:
    - Uterine contraction and retraction.
    - Bearing down efforts by women.
    - Straightening of the fetal ovoid head especially after rupturing of the membrane and full dilatation of the cervix.
     
  3. Flexion:
    The maternal pelvic floor muscles will fight the vertex as it falls into the maternal pelvis. This will induce the fetal head to stretch onto the thorax.
     
  4. Internal rotation:
    The fetus will be in a transverse position at the time of engagement and fall from the vertex into the pelvis. The occiput will be compelled to rotate to the symphysis pubis due to the anatomic shape of the pubococcygeus and iliococcygeus muscles. This is the biggest space of the pelvic floor through which the fetus can pass.
     
  5. Crowing of the head
    After internal rotation of the head, the sub occiput descends till it is beneath the pubic arch. At this stage, the maximum diameter of the head spans the vulval outlet without any recession of the head, which is known as the crowning of the head.
     
  6. Extension:
    The head is delivered via extension via the "a pair of force " hypothesis. This cardinal movement is caused by the combined influences of uterine contractions and the pelvic floor at the time of fetal head delivery.
     
  7. Restitution of the head
    It is the visible passive movement of the head to reverse the neck twist caused by internal rotation of the head. The occiput in a vertex, LOA, returns 1/8 of a circle to the left, back to where it was before internal rotation occurred.
     
  8. External rotation:
    This movement happens as a result of the fetus's head aligning with its spine, which relieves pressure on the fetal head from the mother's pelvis and muscles.
     
  9. Expulsion:
    After the head is delivered, the anterior shoulder will fall beneath the pubic bone. It will follow the external rotation of the fetus's head when it is delivered.

 

Things to remember
  • During labor, the fetus undergoes a series of changes in position, attitude, and presentation.
  • This procedure is required for a successful vaginal birth.
  • To adapt to the mother's pelvic dimensions, the fetus must experience a series of modifications in its presenting part's attitude.
  • This is necessary for fetal descent along the birth canal. In a vertex presentation, the cardinal movements of labor are engagement, descent, flexion, internal rotation, the crowing of the head, extension, restitution of the head, external rotation, and expulsion.
Questions and Answers

Cardinal Movements of Labor are:

To accommodate itself to the maternal pelvic dimensions, the fetus must undergo a series of changes in the attitude of its presenting part. This is required for fetal descent through the birth canal. The cardinal movements of labor in a vertex presentation are as follows:

  • Engagement:

The vertex is engaged when the biparietal diameter is at the level of the pelvic inlet or lower. In clinical practice, this translates into an occiput palpable at 0 stations.

  • Descent:

This cardinal movement will usually happen concomitant with engagement, and is typically documented late in the active phase of the first stage of labor.

Factors Facilitating Descent are :

  • Uterine contraction and retraction.
  • Bearing down efforts by women.
  • Straightening of the fetal ovoid head specially after rupturing of the membrane and full dilatation of the cervix.

Flexion:

As the vertex descends into the maternal pelvis, it will encounter resistance from the maternal pelvic floor muscles. This will cause the flexion of the fetal head onto the thorax.

Internal Rotation:

The fetus will be in a transverse position at the time of engagement and vertex descent into the pelvis. The pubococcygeus and iliococcygeus muscles' anatomical arrangement will lead the occiput to be pushed to rotate toward the symphysis pubis. The largest part of the pelvic floor that the fetus can pass through is here.

Crowing of the Head

After internal head rotation, the sub occiput continues to descend until it is below the public arch. At this point, the vulval outlet is stretched to its maximum diameter without any head recession even after the contraction is finished, a process known as crowing of the head.

Extension:

The "a couple of force" hypothesis is extended in order to deliver the head.

The pelvic floor and uterine contractions work together to produce this cardinal movement just before the fetal head is delivered.

Restitution of the Head

It is the visible passive movement of the head to undo the twist in the neck, that took place during internal rotation of the head. In a vertex, LOA, the occiput restitutes 1/8 the of a circle to the left, back to where it was before internal rotation took place.

External Rotation:

This movement occurs as a consequence of the alignment of the head of the fetus with its spine as the pressure from the maternal pelvis and muscles on the fetal head is alleviated.

Expulsion

After delivery of the head, the anterior shoulder will descend under the pubic bone. As it is delivered, it will follow the external rotation of the head of the fetus.

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