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Mechanism of Normal Labor

 Mechanism of normal labor

Definition: The series of movements that occur on the head in the process of adaptation during its journey through the pelvis is called mechanism of labor.

Mechanism: 

 In normal labor, the head enters the brim more commonly through the available transverse diameter (70%) and to the lesser extent through one of the oblique diameters.

The position is either occipitolateral or oblique occipitoanterior. Left occipitoanterior is more common than right occipitoanterior as the left oblique diameter is encroached by the rectum.

The engaging anteroposterior diameter of the head is either suboccipitobregmatic 9.5 cm or in slight deflexion-the suboccipitofrontal 10 cm. The engaging transverse diameter is biparietal 9.5 cm. As the occipitolateral position is most common, the mechanism of the labor in such position will be described.

The principal movements are:

  1. Engagement
  2. Descent
  3. Flexion
  4. Internal rotation 
  5. Crowning
  6. Extension
  7. Restitution
  8. External rotation
  9. Expulsion of the trunk
1. Engagement

Head brim relation prior to the engagement as revealed by imaging studies shows that due to lateral inclination of the head, the sagittal suture does not strictly correspond with the available transverse diameter of the inlet. Instead, it is either deflected anteriorly toward the pubis symphysis or posteriorly toward the promontary. Such deflection of the head in relation to the pelvis is called Asynclitism

Posterior asynclitism or posterior parietal presentation: When the sagittal suture lies anteriorly, the posterior parietal bone becomes the leading presenting part.



Anterior asynclitism or anterior parietal presentation: Sagittal suture lies more posteriorly with the result the anterior prietal bone becomes the leading presenting part.



Severe degree of asynclitism indicates cephalopelvic disproportion.

2. Descent

Descent is a continuous process. It is slow or insignificant in first stage but pronounced in 2nd stage. It is completed with the expulsion of the fetus. In primigravidae, with prior engagement of the head, there is practically no descent in first stage; while in multiparae, descent starts with engagement. Head is expected to reach the pelvic floor by the time the cervix is fully dilated.

Factors facilitating descent are:

  • Uterine contraction and retraction.
  • Bearing down efforts.
  • Straightening of the ovoid fetal especially after rupture of the membranes.

3. Flexion

As the head meets the resistance of the birth canal during descent, full flexion is achieved. Thus, if the pelvis is adequate, flexion is achieved either due to the resistance offered by the unfolding cervix, the walls of the pelvis or by the pelvic floor.

Flexion is explained by the two-arm lever theory- the fulcrum represented by the occipito-allantoid joint of the head, the short arm extends from the condyles to the occipital protuberance, and the long arm extends from condyles to the chin. When resistance is encountered, by ordinary law of mechanics, the short arm descends and the long arm ascends resulting in flexion of the head.




4. Internal  rotation

The theories which explain the anterior rotation of the occiput are:

Slope of pelvic floor: Two halves of levator ani form a gutter and viewed from above, the direction of the fibers is backward and toward the midline. Thus during each contraction, the head, occiput in particular, in well flexed position, stretches the levator ani, particularly that half which is in relation to the occiput. After the contraction passes off, elastic recoil of the levator ani occurs bringing the occiput forward toward the midline. Process is repeated until the occiput is placed anteriorly. This is called rotation by law of pelvic floor (Hart's rule)



Pelvic shape: Forward inclination of the side walls of the cavity, narrow bispinous diameter and long anteroposterior diameter of the outlet result in putting the long axis of the head to accomodate in the maximum available diameter, i.e. anteroposterior diameter of the outlet leaving behind the smallest bispinous diameter.

Law of unequal flexibility (Sellheim and Moir): The internal rotation is primarily due to inequalities in the flexibility of the component parts of the fetus.

In the occipitolateral position, there will be anterior rotation by two-eighths of a circle of the occiput. Whereas in oblique anterior position, rotation will be one-eighth of the circle forward, placing the occiput behind symphysis pubis. Rotation occurs at the level of pelvic floor.



5. Torsion of the neck

Torsion of the neck is an inevitable phenomenon during internal rotation of the head. If the shoulders remain in the antero-postrior diameter, the neck has to sustain a torsion of two-eighths of a circle corresponding with the same degree of anterior rotation of the occiput. But the neck fails to withstand such major degree of torsion and as such there will be some simultaneous rotation of the shoulders in the same direction to the extent of one-eighths of a circle placing the shoulders to lie in the oblique diameter with one-eighths of torsion still left behind.

Thus, the shoulders move to occupy the left oblique diameter in left occipitolateral position and right oblique diameter in right occipitolateral position. 

In oblique occipitoanterior position, there is no movement of the shoulders from the oblique diameter as the neck sustains a torsion of only one-eighth of a circle.

6. Crowning

After internal rotation of the head, further descent occurs until the subocciput lies underneath the pubic arch. At this stage, the maximum diameter of the head (biparietal diameter) stretches the vulval outlet without any recession of the head even after the contraction is over- called "crowning of the head".



7. Extension

Delivery of the head takes place by extension through "couple of force" theory. The driving force pushes the head in a downward direction while the pelvic floor offers a resistance in the upward and forward direction. The downward and upward forces neutralize and remaining forward thrust helping in extension.

The successive parts of the fetal head to be born through the stretched vulval outlet are vertex, brow, and face. Immediately following the release of the chin through the anterior margin of the stretched perineum, the head drops down, bringing the chin in close proximity to the maternal anal opening.



8. Restitution

It is the visible passive movement of the head due to untwisting of the neck sustained during internal rotation. Movement of restitution occurs rotating the head through one-eighth of a circle in the direction opposite to that of internal rotation. The occiput thus points to the maternal thigh of the corresponding side to which it originally lay.

9. External rotation

It is the movement of rotation of the head visible externally due to internal rotation of the shoulders. As the anterior shoulder rotates toward symphysis pubis from oblique diameter, it carries the head in a movement of external rotation through one-eighth of a circle in the same direction as restitution. The shoulders now lie in the AP diameter. 

The occiput points directly toward the maternal thigh corresponding to the side to which it originally directed at the time of engagement.

10. Birth of shoulders 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. Rest of the trunk is then expelled out by lateral flexion.



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