MECHANISM OF LABOUR
Demonstrate skills in performing mechanism of labor (normal and abnormal)
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)
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.
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
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.
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.
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 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.
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.
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.
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.
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.