MECHANISM OF LABOUR
Objectives:
Demonstrate skills in performing mechanism of labor
(normal and abnormal)
Leaning activities:
1.Pelvis and skull
demonstration
2.Skill in explaining female
pelvis and fetal skull
3.Performing vaginal
examination
4.Mechanism of labor (
normal and abnormal demonstration)
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.
Principles:
1.
Descent takes place
throughout labor
2.
Whichever part leads
and first meets the resistance of the pelvic floor will rotate forwards until
it comes under symphysis pubis
3.
Whatever part merges
from the pelvis will pivot around the pubic bone
LOA/ROA:
Criteria :
Lie: longitudinal
Presentation: cephalic or vertex
Position: right or left occipito-anterior
Attitude: the attitude is one of good/ complete flexion
Denominator: is the occiput
Presenting part: is the posterior part of the anterior parietal bone
The cardinal movements of the labor are
engagement, descent, and flexion, internal
rotation of the head,crowning, extension, external rotation, and
expulsion.
Engagement:
Engaging antero posterior diameter is
suboccipito-bregmatic (9.5cm) and transverse diameter is bi-parietal diameter.
(9.5cm )
In primi gravida, engagement occurs two weeks prior to
the onset of labor and in multi, at the onset of labor.
Descent
With good uterine contractions and maternal bearing
down efforts descent takes place with increasing flexion,
Factors facilitating descent are-
1. Throughout the first stage of labor the forces of
uterine contraction and retraction aid descent.
2. Following rupture of the fore-waters and full
dilatation of the cervix
3. Maternal
bearing down efforts.
Flexion
Flexion increases throughout the labor. The fetal
spine is attached nearer the posterior part of the skull; pressure exerted down
the fetal axis will be more forcibly transmitted to the occiput than the
sinciput. The effect is to increase the flexion which results in smaller
presenting diameters, which will negotiate the pelvis more easily. At the onset
of the labor the sub-occipito-frontal diameter, 10cm, is presenting; with
greater flexion the suboccipetobregmatic diameter, 9.5cm, presents. The occiput
becomes the leading part.
Internal rotation of the head:
During contraction, the leading part descends
downwards on to the pelvic floor. In
well flexed vertex presentation the occiput leads and meets the pelvic
floor first and rotates anteriorly through 1/8th of a circle forward,. Placing the
occiput behind the symphysis pubis. This causes slight twist in the neck of the
fetus, as the head no longer direct alignment with the shoulders. The
antero-posterior diameter of the head now lies in the widest diameter
(antero-posterior) of the pelvic outlet, facilitating an easy escape.
Crowning:
The occiput slips beneath the sub-pubic arch and
crowning occurs. The head no longer recedes between the contractions and the
widest transverse diameter is born.
Extension:
Once the crowning has occurred, the occiput slips
beneath the pubic arch, the sinciput,
face and chin which sweep the perineum and are born by a movement extension.
Restitution:
The twist in the neck of the fetus which resulted from
internal rotation is now corrected by a slight untwisting movement. The occiput
moves 1/ 8 th of a circle towards the side from which it started.
Internal rotation of the shoulders and external
rotation of the head:
As the anterior shoulder rotates towards the symphysis
pubis from the oblique diameter, it carries the head in a movement of external rotation
through 1/ 8th of a circle in the same direction as restitution. The shoulders
now lie in the antero-posterior
diameter.
The occiput points directly towards the maternal thigh
corresponding to the sid which it
originally directed at the time of engagement.
Lateral flexion:
Anterior shoulder escapes below the symphysis pubis
first. By a movement lateral flexion of spine, the posterior shoulder sweeps
over the perineum. Rest of the trunk is then expelled out by lateral flexion.
ROA: occiput points towards the right iliac pubic
eminence and shoulder lies right oblique
diameter
Restitution Occurs towards mother's right side.
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