Mechanism of childbirth

There are four moments of the mechanism of birth. The first moment is the bending of the head; the second is the internal rotation of the head; the third is the extension of the head (the region of the suboccipital fossa is the fixation point - hypomochlion); the fourth is the inner turn of the trunk and the outer turn of the head.

As is known, classical and parallel planes are distinguished in the basin:

• The 1st classical plane goes from the cape to the upper edge of the lone articulation;
• The second classical plane extends from the middle of the inner surface of the pubic articulation to the site of articulation of the II sacral vertebra from III;
• The third classical plane extends from the lower edge of the pubic articulation through the spinous processes of the ischium bones to the sacrococcygeal articulation;
• The fourth classical plane extends from the lower edge of the lone articulation to the top of the coccyx.

The parallel planes proposed by Gojem are also used as objective criteria for stepping the head. Anatomical boundaries of parallel planes are as follows:

• The first plane of entry into the basin runs from the upper edge of the womb along an unnamed line;
• 2nd - from the lower edge of the lona goes parallel to the 1st plane;
• 3rd - passes through the spinous processes of the sciatic bones parallel to the two first planes;
• 4th - from the end of the coccyx parallel to the three upper planes.

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

• 1st classical together with the 1st parallel plane form the entrance to the basin; The 1st classical plane rolls to the narrowest part of the entrance to the pelvis (in a place of a direct size), the size of which will contribute to a certain mechanism of adaptation of the head to the basin;
• The second classical plane represents the widest part of the pelvis. The dimensions of the 2nd classical plane, straight and transverse, are 12.5-13 cm. Finding the base of the large segment of the head on the 2nd classical plane indicates the possibility of turning the head;
• The third classical plane indicates the place of transition of the wide part of the pelvic cavity to the narrow, the place of the onset of the influence of the pelvic floor muscles on the turn of the head;
• 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 view of the occipital presentation.

The head, which is established in the rear view at the entrance to the pelvis, is born in the rear view only in 4%, and in 96% passes into the front view. However, the number of traumatized children in childbirth in the rear view (36%) exceeds the number (4%) of head births in the rear view. Trauma is, apparently, the result of passing the head through the bone pelvis. It is not excluded that this is due to the size of small oblique dimensions of A. Ya. Klassovsky, equal to 8-8.8 cm and which go from the cape to the nameless line of the right and left sides, parallel to the large oblique pelvic dimensions. Thus, the head, entering the entrance to the pelvis in the rear view, is unbent because it meets a serious obstacle (resistance) at the entrance to the pelvis in the region of a small oblique size (8-8.8 cm), inferior in size to the large transverse dimension of the head 9.25 cm). The head, forced to adapt to the entrance to the pelvis in its unfolded state, experiences resistance already from all sides of the entrance to the pelvis. The head is compressed in a straight and transverse dimension, stretching diagonally in the direction of the arrow-shaped seam.

In the forward view of the occipital presentation, a small fontanelle is located below the large fontanelle and is a wired point. In the back view of the occipital presentation, the wire point is the midpoint of the distance between the small and large fontanel. At internal research the big fontanelle - below small or both at one level, the big fontanel - in front (at a front view - a small fontanel is turned anteriorly). The transition from the rear view to the front is due to the fact that the wider occipital part presses on the muscles of the pelvic floor more than the frontal part, as a result of which the head rotates from the rear view to the front and then into the direct dimension of the pelvic outlet (the head thus makes turn on 135 "). However, the second moment is the internal rotation of the head, which can occur in another way: a small fontanelle turns posteriorly (to the sacrum), a large one - to the left articulation.

In foreign literature, the posterior aspect of the occipital presentation is called the "stable position of the head with the back of the head". Clinically, this is characterized by a prolonged subsidence or a stopping of the lowering of the presenting part of the fetus. At the same time, there is a prolonged latent and active phases of labor, a prolonged slowing down phase, but the predominant position is occupied by disorders related to the lowering of the presenting part of the fetus. To suspect an incorrect location of the fetal head should be in cases when it remains at a standing height - 1 or 0 (head small or large segment at the entrance to the pelvis) when the neck is opened for the last few centimeters. This suspicion is all the more reasoned if the presenting part is at a high level of standing and after the full disclosure of the cervix.

Recall that in foreign literature, the location of the presenting part of the fetus (head) is determined by the following numerical designations:

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

Often the stop of further lowering of the presenting part of the fetus is associated with incomplete opening of the cervix. Often, such violations occur with epidural analgesia or with an overdose of sedatives and pain medications. The majority of parturient women have no signs of a narrowed pelvis and therefore, in case of insufficient labor activity, the therapy of choice is the stimulation of labor by the intravenous administration of oxytocin. In many cases, it is followed by a subsequent spontaneous turning of the fetal head with the occiput anteriorly and delivery through the natural birth canal, or the head drops to the level at which the child can be born from the back of the head. It is advisable to produce an episiotomy in the latter case to prevent the perineal rupture.

Some authors recommend, with full disclosure of the uterine throat, to perform epidural analgesia with simultaneous administration of intravenous oxytocin, which gives a high effect for correcting the position of the fetal head from the rear view to the front view of the occipital presentation. In the absence of distress (distress) of the fetus and discrepancy between the size of the pelvis and the fetal head, the II period of labor can last up to 3 hours without any adverse effect on the child's condition. It is desirable to determine the pH of fetal blood, because in the second stage of childbirth there is a progressive decrease in the pH of fetal blood, even in cases when direct electrocardiography gives normal parameters.

When the head is on the pelvic floor, an attempt is made to finger the head back to the head, especially in combination with a slight pressure on the bottom of the uterus as an assistant.

F. Arias recommends the following procedure for finger rotation of the head with the occiput anteriorly:

• the head should be at the level of the pelvic floor and viewed at the entrance to the vagina;
• with the help of the right hand at the left position and the left hand at the right position of the fetus, a lambdoid suture is found and the tip of the middle finger is placed exactly at its angle, and the tip of the index finger is immediately near the middle on the upper part of the lambdoid suture;
• The second hand outside, clenched into a fist, is opposite the front shoulder of the child;
• simultaneously 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 child's shoulders in the transverse direction (counterclockwise) towards the occiput. The pressure opposite to the rotational movement of the fingers located in the vagina leads to the bending of the head and the correction of asynclitism. These two pressures must act simultaneously.

The duration of the second stage of labor, exceeding 3 hours in primiparas and 2 hours in re-births with insufficient advancement (lowering) of the presenting part of the fetus is an indication for the operation of a caesarean section. Cervical forceps should be preferred to cesarean section.

Output obstetrical forceps in the posterior forms of the occipital presentation are imposed exactly as for the forward views: with the straight arrow-suture standing, biparietally on the fetal head and transversely with respect to the pelvis; with oblique position of the swept suture - biparietally on the head and in an oblique pelvis; with a transverse steep suture - in an oblique diameter on the head and in the oblique diameter of the pelvis.

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

The average fluctuations in the mass of the newborn ranged from 282.9 to 519.8 g in the male children of the primiparous. In the case of multiple births, from 340.4 to 519.9 g. In females and newborn females, these deviations from the mean were respectively 357.4-456.3 g and 87.4-476.7 g.

The mass of the body of a newborn child (Campbell et al., 1993)

Term of pregnancy, weeks	Body weight of newborn, g
	From perverse mothers	From many-delivered mothers	From primary mothers	From many-delivered mothers
	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

For the prevention of bleeding in the consecutive and early postpartum periods, the following are recommended: the introduction of uterine contracting agents - intravenous methylergometrine or oxytocin at the time of the eruption of the head or anterior shoulder, emptying the bladder with a catheter, ice onto the uterine projection immediately after the birth of the afterbirth.

[1], [2], [3],