Mechanism Of Normal Labour

Mechanism Of Normal Labour


MECHANISM:

  • Head enters the brim  through transverse diameter (70%) and to a lesser extent through one of the oblique diameters
  • Position:occipitolateral or oblique occipitoanterior(Left>right)
  • The principal movements are
  • Engagement
  • Descent
  • Flexion
  • Internal rotation
  • Crowning
  • Extension
  • Restitution
  • External rotation
  • Expulsion of the trunk

ENGAGEMENT:

Engaging anteroposterior diameter of the head:

  • Either suboccipitobregmatic 9.5 cm (3 3/4″) 
  • In slight deflexion—the suboccipitofrontal (commonest)10 cm (4″).
  • Engaging transverse diameter:Biparietal 9.5 cm (3.74″).
  • In primigravidae, engagement occurs in before the onset of labor
  • In multiparae, engagement may occur in late first stage with rupture of the membranes.
  • One fifth of the head may be palpable per abdomen during engagement of the head
  • Vertex will be at the level of ischial spine during engagement of the head

Asynclitism:

  • Due to lateral inclination of head it is either deflected anteriorly toward symphysis pubis or posteriorly toward sacral promontory.
  • Posterior asynclitism or posterior parietal presentation:
  • Sagittal suture lies anteriorly
  • Posterior parietal bone becomes the leading presenting part
  • Found in primigravidae because of good uterine tone and tight abdominal wall

  • Anterior parietal presentation or anterior presentation:
  • Sagittal suture lies more posteriorly
  • Parietal bone becomes leading presenting part
  • Found in multiparae

Advantages:

  • Helps lesser diameter (super subparietal: 8.5 cm), to cross the pelvic brim instead of larger biparietal diameter
  • Asynclitism is beneficial in the mechanism of engagement of head.
  • Marked and persistent asynclitism is abnormal and indicates cephalopelvic disproportion
DESCENT:
  • Pronounced in second stage
  • Completed with the expulsion of the fetus
  • Primigravidae:no descent in first stage
  • Multiparae, descent starts with engagement

Factors facilitating descent:

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

FLEXION:

  • Due to the resistance offered by unfolding cervix, walls of the pelvis or by the pelvic floor
  • Precedes internal rotation
  • Reduces the shape and size of the plane of the advancing diameter of the head
  • Two-arm lever theory:When resistance is encountered short arm(condyles to the occipital protuberance) descends and the long arm(condyles to chin) ascends resulting in flexion of the head (fulcrum represented by the occipito-allantoid joint of head)

INTERNAL ROTATION: 

Theories that explains anterior rotation of the occiput:

Hart’s rule:

  • Fibers is backward and toward the midline
  • During each contraction head, occiput in particular, in well-flexed position, stretches the levator ani, particularly that half which is in relation to  occiput.
  • After the contraction elastic recoil of the levator ani occurs bringing the occiput forward toward the midline
  • Process is repeated until occiput is placed anteriorly.

Pelvic shape:

  • Forward inclination of side walls of cavity, narrow bispinous diameter and long anteroposterior diameter of outlet result in putting long axis of the head to accommodate in anteroposterior diameter of the outlet.

Law of unequal flexibility (Sellheim and Moir):

  • Internal rotation is primarily due to inequalities in flexibility of component parts of fetus.
  • Occipitolateral position:Anterior rotation by two-eighths of a circle of the occiput
  • Oblique anterior position:Rotation will be one-eighth of a circle forward placing occiput behind the symphysis pubis
  • Mentoposterior position:Rotation is 3/8th of circle 

TORSION OF NECK:

  • Neck has to sustain a torsion of two-eighths of a circle corresponding with same degree of anterior rotation of the occiput with some amount of simultaneous rotation of the shoulders in the same direction to the extent of one-eighth of a circle.
  • Shoulders move to occupy the left oblique diameter in left occipitolateral position and right oblique diameter in right occipitolateral position.
CROWNING:
  • Stretching of  the vulval outlet without any recession of the head (by biparietal diameter) even after the contraction is over called “crowning of the head”

EXTENSION:

  • Delivery of the head takes place by extension through “couple of force” theory.
  • Downward (driving force) and upward forces (pelvic floor resistance) neutralize and remaining forward thrust helping in extension.

RESTITUTION:

  • Visible passive movement of the head due to untwisting of the neck sustained.
  • Rotating the head through one-eighth of circle in the direction opposite to that of internal rotation
  • Occiput points-Maternal thighs
  • Maximum cervical dilatation during labor is 10 cm
  • During active labor, cervical dilatation per hour in primigravida is 1 cm/hr

EXTERNAL ROTATION:

  • Movement of rotation of the head visible externally due to internal rotation of the shoulders
  • Shoulder:A-P diameter
  • Occiput points-Maternal thighs

Exam Question

  • Flexion, Extension & Internal rotation and Descent  are the cardinal movements of the head in normal labour.
  • Suboccipitofrontal is the commonest diameter of engagement
  • Engagement in primigravida occurs at Beginning of labour
  • Engagement of fetal head is When the widest diameter of the presenting part has passed through the pelvic inlet
  • Engagement of foetal head is with reference to Biparietal diameter
  • The cardinal movements during normal labour occur inorder as  Engagement, internal rotation, delivery of head, restitution, external rotation
  • One fifth of the head may be palpable per abdomen during engagement of the head
  • The vertex will be at the level of ischial spine during engagement of the head
  • The biparietal diameter has passed through the pelvic inlet in engagement
  • The internal rotation in mentoposterior position is 3/8th of circle 
  • Maximum cervical dilatation during labor is 10 cm
  • During active labor, cervical dilatation per hour in primigravida is 1 cm/hour
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