The mechanism of normal labor is basically classified into three types;
1.Fetus in utero
2.The definition of the mechanism of labor
3.The cardinal movements of normal labor
1.Fetus in utero
In normal labor, the fetal position in the maternal uterus is like ;
The lie is longitudinal. The presentation is cephalic. Here the presenting part of the fetus is the vertex.
The attitude and denominator are respectively generalized flexion and extension. Also, the positions are like right occiput anterior or left occiput anterior.
What is the diameter of engagement of the pelvis?
In the inlet of the maternal pelvis, the transverse diameter is 13cm.
What is the diameter of engagement of the fetal skull?
The diameter of the fetal skull for engagement is like the sub-occipital bregmatic is 9.5 cm and the sub-occipital frontal is 10cm.
2.What is the definition of the mechanism of labor?
The mechanism of normal labor is defined as the series of movements that occur on the head, in the process of adaptation during its journey through the pelvis.
It is also defined as the series of changes in position and attitude that the presenting part has to make during its passage through the pelvis and pelvic floor during the course of labor.
3.What do you mean by cardinal movements of normal labor?
It is nothing but the entering of the fetal head into the maternal pelvic brim in normal labor. This mainly occurs through the transverse diameter and to a lesser extent through one of the oblique diameters.
Here the most common position is the left occipital anterior. As compared to the right occipital anterior it’s the commonest one as the left oblique diameter is encroached by the rectum.
The diameter of the head, engaged in the anterior posteriorly is either sub-occipital bregmatic or sub-occipital frontal. The sub-occipital bregmatic diameter and sub-occipital frontal are respectively 9.5cm and 10 cm.
Principal movements of normal labor
1.engagement during labor
In the left occipital anterior position (LOA), the vertex of the fetal head enters the pelvic brim along with the occiput. Here occiput is the denominator.
And the occiput is lying with the left iliopectineal eminence and sinciput at the right sacroiliac joint. Like that the sagittal suture of the fetus lying on the right oblique diameter of the maternal pelvis.
Engagement in ROA
In the right occipital anterior position(ROA), the vertex of the fetal head is situated in the left oblique diameter of the pelvis. And the occiput is lying in the right anterior quadrant of the maternal pelvis.
In this phase, the engaging fetal head diameter is biparietal 9.5cm. And the anterior-posterior bregmatic is also 9.5cm. Both lie on the same plane.
Primigravida and multipara
2.Descent during labor
The descent is a continuous process. It is continued until there is no bony tissue obstruction.
Basically, the process of descent is slow in the first stage of labor. It is progressive during the second stage. This is concluded with the delivery of the fetus.
The factors responsible for descent are;
Contraction and retraction of the uterus
Bearing down effects
Straightening of fetal ovoid especially after rupturing of the membrane
The fetal head is flex due to the resistance. This resistance is provided by the birth canal during descent.
The factors responsible for flexion during normal labor are;
Wall of pelvis
In occipital lateral position, there will be anterior rotation by 2/8 that of a circle of occiput whereas in oblique, the rotation is ⅛ the of a circle placing the occiput behind symphysis pubis.
In crowing, the maximum diameters of the head stretch the vulva outlet without a recession of the head even after the contraction is over called “ crowing of the head”.
Delivery of the head takes place by extension through the “couple of force” theory. In this theory, the driving force pushes the head in a downward direction.
While the pelvic floor offers resistance in the upward and forward direction. The downward and upward force neutralizes and the forward thrust helps in extension.
The successive part of the fetal head is born through stretched vulva outlets are the vertex, brow, and face.
It is the visible passive movements of the head. it occurs mainly due to the untwisting of the neck sustained during internal rotation. Movements of restitution occur by rotating the head through 1/8th a circle in the direction opposite to the internal rotation.
8.External rotation with the internal rotation of shoulders
It is this movement of the head, visible externally due to the internal rotation of the shoulder. As the anterior shoulder rotates towards the symphysis pubis.
This process helps the fetal head to move externally through 1/8th of a circle. it occurs in the same direction as restitution. The shoulders now lie in anteroposterior diameters.
9.Expulsion or the birth of the shoulder and trunk
After the external rotation of the fetal head, the shoulders are in the anteroposterior diameter. Again descent takes place. It helps the anterior shoulder escape below the symphysis pubis.
The shoulder comes out due to the lateral flexion of the maternal spine. Like that, the posterior shoulder also escapes over the perineum. Afterward, the trunk is expelled out by lateral flexion.