Mechanism of Normal Labour

Subject: Midwifery II (Theory)

Overview

The set of motions a fetus makes during delivery is referred to as a labor mechanism. Engagement is the process by which the largest portion of the fetus passes through the pelvic inlet. Descent describes the movement of the presenting component via the pelvis as it moves downhill. Similar to flexion, internal rotation is a passive motion brought on by the shape of the pelvis and the pelvic floor muscles' resistance. Extension happens when the fetus reaches the introitus' level. An action similar to internal head rotation is internal shoulder rotation. When the body is being expelled, the spine bends sideways to accommodate the birth canal's curve. This movement is known as lateral flexion.

MECHANISM (CARDINAL MOVEMENTS) OF LABOR

Definition: The term "labor mechanism" refers to the series of motions that the head undergoes during the adaptation process as it passes through the pelvis.

Principles common to all mechanism:

  • Descent takes place
  • The part that leads and first encounters the pelvic floor's resistance will rotate forward until it passes under the symphysis pubis.
  • The pubic bone will serve as the pivot point for whatever comes out of the pelvis.

Six considerations for normal labor:

  • It is a longitudinal lie.
  • The appearance is cephalic.
  • The occipitoanterior position is either right or left.
  • One of the best flexions is the attitude.
  • The occiput is the denominator.
  • The posterior portion of the anterior parietal bone is the portion that is visible.

As the left occipito - anterior and occipito - lateral are the commonest, the mechanism of labor in such position is as follows:

Cardinal movements are:

  • Engagement
  • Descent
  • Flexion
  • Internal rotation of head
  • Crowning of the head
  • Extension of the head
  • Restitution
  • Internal shoulder rotation and external head rotation lateral flexion
  • Delivery of the shoulder and trunk

Engagement

Engagement is the process by which the largest portion of the fetus passes through the pelvic inlet. In nulliparous women, this typically occurs two to three weeks prior to labor, and in multiparous women, it may happen at any point before or after labor begins.

Descent

Descent describes the movement of the presenting component via the pelvis as it moves downhill. The process of the fetal descent is not steady or continuous. The two stages of labor's deceleration and the third stage are when falls occur at the highest rate.

Flexion

As it passes through the pelvis, the fetal head flexes, bringing the chin into contact with the fetal chest. As a result, a smaller structure emerges that can fit through the pelvis of the mother. The anterior fontanel gets further away and more challenging to feel as the occipital (posterior) fontanel creeps into the birth canal. Transverse occiput remains the fetal posture.

Internal Rotation

Internal rotation is the rotation of the presenting component from its initial position—typically transverse in relation to the birth canal—to the anteroposterior position as it moves through the pelvis. Similar to flexion, internal rotation is a passive motion brought on by the shape of the pelvis and the pelvic floor muscles' resistance.

Crowning of the Head

After the discomfort episode has passed, the head is termed to have crowned when it enlarges the vaginal opening without receding into the vagina.

Extension

Extension happens when the fetus reaches the introitus' level. This fall brings the base of the occiput into contact with the inferior border of the symphysis pubis. At this moment, the birth canal slopes upward. During extension, the fetal head emerges and revolves around the pubis symphysis. The pelvic floor muscles' upward forces and the downward forces applied to the fetus by uterine contractions and maternal expulsive efforts are what propel the motion.

Restitution

The neck twist brought on by internal head rotation is reversed by the visibly passive movements of the head. When an internal rotation occurs, the occiput of a vertex, LOA, returns 1/8th of a circle to the left. This movement indicates whether the position is right or left, and lets the midwife know whether she is giving birth to an LOA or a ROA. The occiput then points to the maternal thigh on the opposite side from where it originally rested. She can deliver the baby on her shoulder more easily without injuring her perineum.

Internal Rotation of the Shoulder

This motion resembles an internal head rotation. In LOA, the shoulder is in the left oblique diameter of the pelvic cavity. The anterior shoulder rotates forward when it reaches the right side of the pelvic floor, placing the shoulders inside the anteroposterior diameter of the exit. When the uterus contracts after the head is born, this should take place.

External Rotation

After responding (expanding), the fetal head rotates to the proper anatomic position in relation to the fetal torso; the rotation is to the left or right depending on the orientation of the fetus. Another passive movement, this one is regulated by the basal tone of the fetal musculature and results from the release of forces placed on the fetal head by the mother's bony pelvis and its musculature.

Lateral Flexion of the Body

As the body is emptied to conform to the curve of the birth canal, there is a sideways bending of the spine known as lateral flexion. Ordinarily, the anterior shoulder is the first to develop during birth. The posterior shoulder rests on the perineum, while the anterior shoulder lies beneath the sub-pubic arch. As opposed to if both shoulders had been born at the same moment, this permits a smaller dimension to enlarge the vaginal opening. By lateral flexion, the spine bends sideways through the concave birth canal to give birth to the rest of the body.
Things to remember
  • The fetus goes through a number of position, attitude, and presentational changes during labor.
  • For a vaginal birth to be successful, this process is necessary.
  • The fetus must undergo a number of changes in the attitude of its presenting part in order to adjust to the mother's pelvic dimensions.
  • Fetal descent through the birth canal requires this. The engagement, descent, flexion, internal rotation, crowning of the head, extension, restitution of the head, external rotation, and expulsion are the cardinal movements of labor in a vertex presentation.
Questions and Answers

Mechanism of Normal Labor

  • Engagement

Engagement is the descent of the widest part of the fetus through the pelvic inlet. This normally occurs 2-3 weeks before labor in nulliparous women and may occur anytime before or after an onset of labor in multiparous women.

  • Descent

Descent refers to the downward passage of the presenting part through the pelvis. Descent of the fetus is not a steady, continuous process. The greatest rate of descent occurs during the deceleration phase of the first stage and during the second stage of labor.

  • Flexion

The fetal head bends such that the chin touches the fetal chest as it descends through the pelvis. Functionally, this results in a smaller structure that can fit through the pelvis of the mother. The anterior fontanel becomes farther away and more challenging to feel as flexion takes place because the occipital (posterior) fontanel slides into the center of the birth canal. Occipital transverse remains the fetal position.

  • Internal Rotation

Internal rotation is the movement of the presenting part through the pelvis from its initial position, which is typically transverse with respect to the birth canal, to the anteroposterior position. Similar to flexion, internal rotation is a passive motion brought on by the structure of the pelvis and the pelvic floor muscles' resistance.

  • Crowning of the Head

The head is said to be crowned when it distends the vaginal opening, without retracting inside the vagina after the episode of pain is over.

  • Extension

Once the fetus has sunk to the level of the introitus, extension takes place. The inferior border of the symphysis pubis comes into touch with the base of the occiput as a result of this descent. The birth canal slopes upward at this point. By extension, the fetal head emerges and revolves around the symphysis pubis. The downward force applied to the fetus by uterine contractions and maternal expulsive efforts as well as the upward forces applied by the muscles of the pelvic floor are what cause this motion.

  • Restitution

The neck twist that resulted from the head's internal rotation is being undone by the visible passive motions of the head. The occiput in a vertex, LOA, returns 1/8th of a circle to the left, to its original position before internal rotation occurred. This movement indicates whether the position is right or left and lets the midwife know whether she is giving birth to a LOA or an ROA. The occiput then points to the maternal thigh on the opposite side from where it originally lay. She can more easily control the shoulder's birth without cutting her perineum.

  • Internal Rotation of Shoulder

This is a movement similar to the internal rotation of the head. The shoulder in LOA is in the left oblique diameter of the pelvic cavity. The anterior shoulder reaches the right side of the pelvic floor and rotates forwards bringing the shoulders into the anteroposterior diameter of the outlet. This should take place with the uterine contraction which occurs after the head has been born.

  • External Rotation

After the fetal head reflexes (extends), it rotates to the correct anatomic position in relation to the fetal torso; left or right rotation depends on the orientation of the fetus. This is again a passive movement resulting from a release of the forces exerted on the fetal head by the maternal bony pelvis and its musculature and mediated by the basal tone of the fetal musculature.

  • Lateral Flexion of the Body

When the body is being evacuated, the spine bends sideways to fit to the shape of the birth canal. This process is known as lateral flexion. The anterior shoulder normally comes into the world first as the shoulders develop in order. The posterior shoulder crosses the perineum, while the anterior shoulder slides beneath the subpubic arch. This makes it possible to expand the vaginal orifice with a smaller diameter than if both shoulders had been born at once. The spine bends sideways through the curved birth canal, allowing the remainder of the body to be born by lateral flexion.

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