Mechanism of labor

There are four moments of the birth mechanism. The first moment is flexion of the head; the second is internal rotation of the head; the third is extension of the head (the suboccipital fossa is the point of fixation - hypomochlion); the fourth is internal rotation of the torso and external rotation of the head.

As is known, in the pelvis there are classical and parallel planes:

• The 1st classical plane goes from the promontory to the upper edge of the pubic symphysis;
• The 2nd classical plane goes from the middle of the inner surface of the pubic symphysis to the place of articulation of the 2nd sacral vertebra with the 3rd;
• The 3rd classical plane runs from the lower edge of the pubic symphysis through the spinous processes of the ischial bones to the sacrococcygeal joint;
• The 4th classical plane goes from the lower edge of the pubic symphysis to the apex of the coccyx.

The parallel planes proposed by Godge are also used as objective criteria for the staged advancement of the head. The anatomical boundaries of the parallel planes are as follows:

• The 1st plane of the entrance to the pelvis goes from the upper edge of the pubis along the nameless line;
• 2nd - from the lower edge of the pubis runs parallel to the 1st plane;
• 3rd - passes through the spinous processes of the ischial bones parallel to the first two planes;
• 4th - from the end of the coccyx goes parallel to the three planes located above.

The anatomical boundaries of the parallel and classical planes of the pelvis do not coincide:

• The 1st classical plane together with the 1st parallel plane form the entrance to the pelvis; the 1st classical plane rolls onto the narrowest part of the entrance to the pelvis (in the place of the direct size), the size of which will contribute to a certain mechanism of adaptation of the head to the pelvis;
• The 2nd classical plane is the widest part of the pelvis. The dimensions of the 2nd classical plane, straight and transverse, are 12.5-13 cm. The location of the base of the large segment of the head on the 2nd classical plane indicates the possibility of turning the head;
• The 3rd classical plane indicates the place where the wide part of the pelvic cavity transitions into the narrow part, the place where the influence of the pelvic floor muscles on the rotation of the head begins;
• The 4th classical plane indicates the size and shape of the pelvic outlet.

It is important to take into account the differences in the mechanism of labor in the anterior and posterior types of occipital presentation.

The head, established in the posterior view at the pelvic entrance, is born in the posterior view only in 4%, and in 96% it passes into the anterior view. However, the number of injured children during birth in the posterior view (36%) exceeds the number (4%) of births of heads in the posterior view. Traumatism is, apparently, the result of the head passing through the bony pelvis. It is possible that this is due to the size of the small oblique dimensions of A. Ya. Krassovsky, equal to 8-8.8 cm and which go from the promontory to the innominate line of the right and left sides, parallel to the large oblique dimensions of the pelvis. Thus, the head, entering the pelvic entrance in the posterior view, is straightened due to the fact that it encounters a serious obstacle (resistance) when entering the pelvis in the area of the small oblique size (8-8.8 cm), inferior in size to the large transverse size of the head (9.25 cm). The head, forced to adapt to the entrance to the pelvis in an extended state, experiences resistance from all sides of the entrance to the pelvis. The head is compressed in the direct and transverse dimensions, stretching in the diagonal direction towards the sagittal suture.

In the anterior view of the occipital presentation, the occipital fontanelle is located below the large one and is the reference point. In the posterior view of the occipital presentation, the reference point is the middle of the distance between the small and large fontanelles. During internal examination, the large fontanelle is below the small one or both are at the same level, the large fontanelle is in front (in the anterior view, the small fontanelle is facing forward). The transition from the posterior view to the anterior view occurs due to the fact that the wider occipital part presses on the muscles of the pelvic floor more strongly than the frontal part, as a result of which the head turns from the posterior view to the anterior one, and then to the direct size of the pelvic outlet (the head turns by 135"). However, the second moment is the internal rotation of the head, which can occur differently: the small fontanelle turns back (towards the sacrum), the large one - to the pubic symphysis.

In foreign literature, the posterior type of occipital presentation is called "a stable position of the head with the occiput posteriorly". Clinically, this is characterized by a prolonged descent or cessation of the descent of the presenting part of the fetus. At the same time, a prolonged latent and active phase of labor, a prolonged deceleration phase are observed, but the predominant place is occupied by disorders associated with the descent of the presenting part of the fetus. An incorrect position of the fetal head should be suspected in cases where it remains at the standing height of 1 or 0 (the head with a small or large segment at the entrance to the pelvis) when the cervix opens for the last few centimeters. This suspicion is even more justified if the presenting part is at a high standing level and after the cervix has fully opened.

Let us recall that in foreign literature the location of the presenting part of the fetus (head) is determined by the following digital designations:

• -3 - head above the entrance to the small pelvis;
• -2 - the head is pressed against the entrance to the small pelvis;
• -1 - head with a small segment at the entrance to the pelvis;
• 0 - head with a large segment at the entrance to the pelvis;
• + 1 - the head is in the wide part of the pelvic cavity;
• + 2 - the head is in the narrow part of the pelvic cavity.

Often, the cessation of further descent of the presenting part of the fetus is associated with incomplete dilation of the cervix. Frequently, such disorders occur with epidural analgesia or with an overdose of sedatives and painkillers. Most women in labor do not have signs of a contracted pelvis, and therefore, in case of insufficient labor, the therapy of choice is stimulation of labor with intravenous oxytocin. In many cases, this is accompanied by subsequent spontaneous rotation of the fetal head with the occiput anteriorly and labor through the natural birth canal, or the head descends to a level at which the child can be born with the occiput posteriorly. In the latter case, it is advisable to perform an episiotomy to prevent perineal rupture.

Some authors recommend that epidural analgesia with simultaneous intravenous administration of oxytocin be performed with full dilation of the cervical os, which has a high effect on correcting the position of the fetal head from the posterior view to the anterior view of the occipital presentation. In the absence of fetal distress and discrepancy between the sizes of the pelvis and the fetal head, the second stage of labor can last up to 3 hours without any adverse effect on the child's condition. It is advisable to determine the pH of fetal blood, since in the second stage of labor there is a progressive decrease in fetal blood pH, even in cases where direct electrocardiography gives normal parameters.

When the head is located on the pelvic floor, an attempt to finger-rotate the head with the occiput forward is effective, especially in combination with light pressure on the bottom of the uterus by an assistant.

F. Arias recommends the following technique of finger rotation of the head with the occiput forward:

• the head should be at the level of the pelvic floor and visible at the entrance to the vagina;
• using the right hand for the left position and the left hand for the right position of the fetus, find the lambdoid suture and place the tip of the middle finger exactly at its corner, and the tip of the index finger directly near the middle finger on the upper part of the lambdoid suture;
• the second hand on the outside, clenched into a fist, is placed opposite the child’s front shoulder;
• At the same time, two fingers placed on the lambdoid suture create a constant rotational movement in the direction of the right angle to the sagittal suture (clockwise), and with the fist of the other hand push the baby's shoulder in a transverse direction (counterclockwise) towards the back of the head. Pressure opposite to the rotational movement of the fingers located in the vagina leads to flexion of the head and correction of asynclitism. These two pressures must act simultaneously.

Duration of the second stage of labor exceeding 3 hours in primiparous women and 2 hours in multiparous women with insufficient advancement (descent) of the presenting part of the fetus is an indication for performing a cesarean section. Preference should be given to abdominal forceps for cesarean section.

The exit obstetric forceps for posterior types of occipital presentation are applied in the same way as for anterior types: with a direct position of the sagittal suture - biparietal to the fetal head and transversely in relation to the pelvis; with an oblique position of the sagittal suture - biparietal to the head and in the oblique diameter of the pelvis; with a transverse position of the sagittal suture - in the oblique diameter to the head and in the oblique diameter of the pelvis.

It is important to take into account current data on the weight of the fetus and newborn, taking into account the gestational age and the sex of the child, as well as parity.

Average fluctuations in the weight of a newborn ranged from 282.9 to 519.8 g for males in primiparous women. For multiparous women, it ranged from 340.4 to 519.9 g. For female fetuses and newborns, these deviations from the average were 357.4-456.3 g and 87.4-476.7 g, respectively.

Newborn birth weight (Campbell et al., 1993)

Gestational age, weeks	Newborn body weight, g
	From first-time mothers	From mothers who have given birth to many children	From first-time mothers	From mothers who have given birth to many children
	Boys		Girls
32	1905	2050	1505	1865
33	1950	1910	2000	2040
34	2320	2390	2020	2080
35	2525	2595	2340	2425
36	2650	2700	2600	2580
37	2865	2970	2850	2905
38	3070	3210	2990	3080
39	3280	3400	3125	3260
40	3390	3540	3270	3380
41	3495	3630	3380	3480
42	3500	3490	3390	3405

To prevent bleeding in the placental and early postpartum periods, the following is recommended: administration of uterine contraction agents - intravenous methylergometrine or oxytocin at the moment of cutting of the head or anterior shoulder, emptying of the bladder using a catheter, ice on the area of the projection of the uterus immediately after the birth of the placenta.

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