Mechanism of labor
The mechanism of labor is defined as a position movement carried out by the fetus to adjust to the mother’s pelvis. The main movements of the mechanism of labor are engagement, head loss, flexion, internal rotation (inner rotation), extension, expulsion, external rotation (outer rotation). In short, the mechanism of labor is successive fetal movements to adjust to the birth canal.
Here we have submitted the main movements mechanism of labor:
The fetus has been continuously decreasing in the birth canal since the third trimester of pregnancy. This process includes the entry of the largest part of the fetal head into the upper pelvic floor which in primigravida. This usually occurs at 36 weeks of gestation and in 38 weeks of multigravida. The entry of the head into the upper pelvic floor usually with sagittal sutures across the back (Example: if the left back palpation is obtained, the sagittal suture will be felt cross left or 3 o’clock position or vice versa if the right-back is sagittal suture to the right or position 9 o’clock and at that moment the lightly flexed head.
If the sagittal suture is in the anteroposterior diameter of the upper pelvic floor, the entry of the head will be difficult because it occupies the smallest size of the upper pelvic floor. If the sagittal suture is in the position in the middle of the birth canal which is right between the symphysis and promontorium, it is said in the position of “synclitismus” at the position of the front and rear synclitismus os parietale as high. If the sagittal suture is somewhat closer to the symphysis or slightly back near the promontorium, what we are dealing with is the position of “asynclitismus”.
Posterior Acynclitismus is the position of the sagittal suture close to the symphysis and the rear parietale os. It is lower than the front os parietale. Anterior Acynclitismus is the position of the sagittal suture approaching the promontorium so that the front parietale os becomes lower than the rear os parietale.
When the head enters the upper pelvic floor, it usually stays in the position of mild posterior asynclitismus. When the head of the fetus enters the upper pelvic floor, it will be fixed and later called an engagement.
The advance of the fetal head
At the primigravida, the advance of the head occurs after the head enters the pelvic cavity and usually starts at the second time. In multigravida, the progress of the head and the entry of the head in the pelvic cavity occur together. The head advances together with other movements. Later they are called as flexion, inner rotation, and extension. The head movement is caused by:
- Intrauterine fluid pressure
- Direct pressure of the uterine fundus by the buttocks
- Strength of straining
- Straighten the baby’s body by changing the shape of the uterus
Flexion is the position of the fetal head entering the pelvic space with the smallest size, called the sub-occipital bregmatikus diameter (9.5 cm) replacing the sub-occipital frontalis (11 cm). Flexion is caused by the pushed forward fetus instead held on the edge of the upper pelvic floor, cervix, pelvic wall or pelvic floor. Arriving at the pelvic floor the head of the fetus is in a position of maximum flexion. The head down meets the pelvic diaphragm that runs from the top back to the front. As a result, the combination of the elasticity of the pelvic diaphragm and intrauterine pressure caused by repetitive pressure, the rotating head is referred to as the inner rotation. With the presence of detention from the getting bigger pelvic floor, the head of the fetus will decrease and become more flexible so that the fetal chin presses the chest and back of the head (occiput) to the bottom. This condition is called maximum flexion.
- Inner rotation
The lower the fetal head position is in the birth canal. Further, the fetal head will rotate in such a way until the longest diameter of the pelvic cavity or posterior Antero diameter will adjust to the smallest transverse diameter (oblic) upper pelvic floor, and then the smallest antero posterior lower pelvic floor.
The inner rotation is necessary for the birth of the head because this rotation is an attempt to adjust the position of the head with the shape of the birth canal, especially the shape of the middle and lower pelvis. This occurs together with the progress of the head and does not occur before the head reaches Hodge III, sometimes it only occurs after the head reaches the pelvic floor.
The reasons for the rotation in:
At the location of flexion, the head is the lowest part of the head. Then the lowest part of the head looks for the prisoner that is at least on the upper front where there is a genital hiatus between left and right levator ani muscles. The largest size of the middle pelvic plane is called as the anteroposterior diameter.
After the rotation round finished and the head reaches the pelvic floor, there is an extension or head deflection. As a result, the birth axis of the Down Floor of the Pelvis points forward and upward, so that the head must extend to pass it. In rotation, the small fontanel will rotate towards the front, so that at the pelvic floor the small fontanel is under the symphysis, with sub-occiput as the head hypomoclion performs a deflection movement to be born. At the moment, more vulva is opening and the head of the fetus becomes more visible. The perineum becomes wider and thinner, the anus opens the rectal wall. With his strength and straining strength, the bregmatikus, forehead, face, and finally the chin will appear with the extension movement. After the head is born, the head immediately rotates. Later it is called the outer rotation.
- Outer rotation
Outer rotation is the movement of the head that rotates when the head is outside the birth canal, usually, the head will face right or left of the mother’s back. Head adjusts to the long axis of the shoulder, so the long axis of the shoulder and the long axis of the fetus is in a straight line. The helper must wait for the swivel head to pack outside before giving birth to the baby’s shoulders, body, and legs.
After the external rotation, the front shoulder is below the symphysis and becomes hypomochlion for the birth of the back shoulder. After the baby’s shoulders are born, then the entire body of the baby is born in the direction of the birth axis. With effective contractions, adequate head flexion, and an average size fetus, most posterior positions rotate rapidly as soon as they reach the pelvic floor. As a result, labor is not so long but in about 5-10% of cases, this favorable situation does not occur. For example, poor contractions or wrong or both flexion of the head, rotation may not be perfect or may not occur at all, especially if the fetus is large.