Mechanism of Normal Labor

Mechanism of Normal Labor-

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.

 The head enters the brim more commonly through the available transverse diameter (70%) and to a lesser extent through one of the oblique diameters. Accordingly, the position is either occipitolateral or oblique occipitoanterior. Left occipitoanterior is little more common than right occipitoanterior as the left oblique diameter is encroached by the rectum.


  • The measurement of the progress of descent in centimetre’s above or below the midplane from the presenting part to the ischial spine    
  • Station  0- At ischial spine
  • Minus station- Above ischial spine
  • Plus station- Below ischial spine
  • Engagement- When the widest   diameter of the presenting part  has passed the inlet; corresponds to a 0 station II.          

The principal movements are-

  • Engagement
  • Descent
  • Flexion
  • Internal rotation
  • Crowning
  • Extension
  • Restitution
  • External rotation
  • Expulsion of the trunk.

A. Engagement-

Passage of the widest diameter of the presenting part to a level below the plane of the pelvic inlet. The sagittal suture lies more posteriorly with the result that the anterior parietal bone becomes the leading presenting part and is then called anterior parietal presentation or anterior asynclitism. In primigravidae, engagement occurs in a significant number of cases before the onset of labor while in multiparae, the same may occur in late first stage with rupture of the membranes.

Advantages of Asynclitism –

  •  Engagement of head with asynclitism, the two parietal eminences cross the brim one at a time. This helps lesser diameter (super subparietal: 8.5 cm), to cross the pelvic brim instead of larger biparietal diameter (9.5 cm) for engagement in synclitism.
  • Asynclitism is beneficial in the mechanism of engagement of head.
  • Marked and persistent asynclitism is abnormal and indicates cephalopelvic disproportion.

B. Descent-

Downward passage of the presenting part through the pelvis. It is slow or insignificant in first stage but pronounced in second 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.

Factors facilitating descent are-

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

C. Flexion-

Flexion of the fetal head occurs passively as the head descends due to the shape of the bony pelvis and the resistance offered by the soft tissues. The head meets the resistance of the birth canal during. In 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 a circle forward, placing the occiput behind the symphysis pubis. There is always an accompanying movement of descent with internal rotation. Thus, prerequisites of anterior internal rotation of the head are well-flexed head, efficient uterine contraction, favorable shape at the midpelvic plane, and tone of the levator ani muscles. 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 essential for descent, since it reduces the shape and size of the plane of the advancing diameter of the head.

D. Internal rotation-

Rotation of the presenting part from its original position to the anterior position as its passes through the pelvis. Internal rotation is a turning forward of whatever part of the fetus reaches the gutter shaped pelvic floor first.

Theories which explain the anterior rotation of the occiput are-

  • Slope of pelvic floor-The gutter like pelvic floor helps rotation. Levator ani muscles, pelvic body.
  • Also called the rotation by law of pelvic floor floor (Hart’s rule)
  • Resistance-An important determinant of rotation.

E. 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”.

F.  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.

G. 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.

H. 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 the symphysis pubis from the 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 anteroposterior diameter. The occiput points directly toward the maternal thigh corresponding to the side to which it originally directed at the time of engagement.

I. Expulsion –

After the shoulders are positioned in anteroposterior 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.