During childbirth, the fetus passes to the exit from the birth canal, performing translational and rotational movements. The complex of such movements is the biomechanism of childbirth. The presentation of the fetus largely determines the complexity of childbirth. More than 90% of cases are occiput presentation of the fetus.
Biomechanism in primiparas
According to research, in primiparas, the head moves forward a little during pregnancy. The degree of this progress depends on the ratio of the size of the fetal head and the mother's pelvis. For some, the fetus stops its movement at the entrance, and for some, already in the expanded part of the pelvic cavity. When labor begins, the head resumes its advancement when the first contractions appear. If the birth canal interferes with the progress of the fetus, then the biomechanism of childbirth in the anterior view of the occipital presentation occurs in the area of the pelvis where the obstacle is encountered. If the birth proceeds normally, then the biomechanism turns on when the head passes the border between the wide and narrow part of the pelvic cavity. To cope with the obstacles that have arisen, uterine contractions alone are not enough. Attempts appear, pushing the fetus along the way to the exit from the birth canal.
In most cases, the biomechanism of labor in anterior occipital presentation is activated at the stage of expulsion, when the head passes into the narrow part of the pelvic cavity from the wide one, although in primiparous everything can begin at the moment of disclosure, when the fetal head is in the entrance.
During the process of fetal expulsion, the fetus and uterus constantly interact with each other. The fetus tries to stretch the uterus in accordance with its shape and size, while the uterus tightly covers the fetus and amniotic fluid, adapting it to its shape. As a result of such actions, the fetal egg and the entire birth canal achieve the most complete correspondence to each other. This is how the preconditions for the expulsion of the fetus from the birth canal arise.
Moment division
The biomechanism of labor in anterior occiput presentation is conditionally divided into four points:
- flexion of the head;
- its inner turn;
- head extension;
- internal torso rotation combined with external head rotation.
Moment One
Bending the head is that under the influence of intrauterine pressure, the cervical spine bends, bringing the chin closer to the chest, and lowering the back of the head down. In this case, the small fontanel is located below the large one, gradually approaching the wire line of the pelvis, and this part becomesthe lowest part of the head.
The benefit of this flexion is that it allows the head to overcome the smallest pelvic cavity. The straight size of the head is 12 cm, and the small oblique resulting from flexion is 9.5 cm. True, during the normal course of childbirth, there is no need for such a strong bending of the head: it bends as much as it needs to go from wide to narrow pelvic cavity. Maximum flexion of the fetal head is required only in situations where the birth canal is not wide enough to allow the head to pass through. This happens when the pelvis is too narrow, and also in the case of posterior occipital presentation.
Bending is not the only movement of the fetus in this moment of the biomechanism of labor. At the same moment, there is a translational movement of the head along the birth canal, and after the end of flexion, its internal rotation begins. So at the first moment of the biomechanism of childbirth, there is a combination of translational movement with flexion and rotation. However, since the most pronounced movement is the flexion of the head, the name of the first moment reflects this fact.
Moment two
Internal rotation of the head is a combination of its translational movement with internal rotation. It begins when the head is bent and settled at the pelvic inlet.
The fetal head, moving progressively in the pelvic cavity, encounters resistance to furthermovement and begins to rotate around the longitudinal axis. There is a kind of screwing of the head into the pelvis. This occurs most often when it passes from a wide to a narrow part of the pelvic cavity. The back of the head slides along the wall of the pelvis, approaching the pubic joint. This moment can be fixed by watching how the position of the swept seam changes. Before turning, this suture is located in the small pelvis in a transverse or oblique size, and after turning it is located in a direct size. The end of the rotation of the head is marked when the sagittal suture is established in a straight size, and the suboccipital fossa assumes a position under the pubic arch.
Moment three
Extension of the head. The head continues to move along the birth canal, gradually beginning to unbend. In normal delivery, extension is performed at the exit of the pelvis. The back of the head comes out from under the pubic arch, and the forehead protrudes beyond the coccyx, protruding the back and front of the perineum in the form of a dome.
The suboccipital fossa rests on the lower edge of the pubic arch. If at first the extension of the head was slow, at this stage it accelerates: the head unbends literally in a few attempts. The head penetrates through the vulvar ring along its small oblique size.
In the process of extension, the crown, frontal region, face and chin appear in turn from the birth canal.
Moment four
External rotation of the head with internal rotation of the torso. While the head follows along the soft tissues of the pelvicexit, the shoulders are screwed into the pelvic canal. The born head receives the energy of this rotation. At this point, the back of the head turns towards one of the mother's thighs. The front shoulder comes out first, followed by a slight delay due to the bending of the coccyx, and the back shoulder is born.
The birth of the head and shoulders sufficiently prepares the birth canal for the rest of the body to emerge. Therefore, this stage is quite easy.
The considered biomechanism of labor in anterior occiput presentation for primiparous is completely true for multiparous. The only difference is that in those giving birth again, the beginning of the biomechanism falls on the period of exile, when the waters broke.
Actions of obstetricians
In addition to the biomechanism, it is necessary to use obstetric assistance in childbirth.
You can't rely on nature for everything. Even if a woman has a relatively regular birth in the occipital presentation, the help of an obstetrician may be needed.
- The first moment. Protection of the perineum, preventing premature extension. Palms need to hold the head, preventing movement during attempts and increasing flexion. It is necessary to strive to ensure that the bending is not maximum, but such that it is genetically necessary. There is no need to intervene unless absolutely necessary. The child is usually able to adjust itself to the birth canal. Very many complications and birth injuries are caused precisely by obstetric benefits during childbirth, and not by the birth itself. More often, the child is injured not from the perineum of the woman in labor, but from the hands of the midwife,protecting the crotch.
- The second moment - in the absence of attempts to remove the head from the genital slit. If the head comes out at the maximum of attempts, it strongly presses on the genital gap.
The order is this. With the completion of the effort, the vulvar ring is gently stretched with the fingers of the right hand over the emerging head. Stretching is interrupted with the beginning of a new attempt.
These actions, aimed at obstetric benefits, must be alternated until the head of the parietal tubercles approaches the genital opening, when the compression of the head increases and the stretching of the perineum increases. As a result, the risk of injury to the head of the fetus and the woman in labor increases.
The third point is to reduce the tension of the perineum as much as possible to increase the compliance of the penetrating head. The obstetrician gently presses with his fingertips on the tissues surrounding the genital opening, directs them towards the perineum, which reduces its tension.
The fourth point is the adjustment of attempts. The time of appearance of the parietal tubercles of the head in the genital gap increases the risk of perineal rupture and traumatic compression of the head.
Equally greater danger is the complete cessation of attempts. Breathing plays an important role in this. The woman in labor is told to breathe deeply and often with her mouth open to ease the efforts. When the need arises in an attempt, the woman in labor is forced to push a little. By the method of initialization and termination of attempts, the midwife controls the birth of the head at the most crucial time.
The fifth moment is the appearance of the shoulders and torso. After the head comes out, the woman in labor needs to push. Hangersare born, as a rule, without the help of an obstetrician. If this does not happen, the head is captured by hand. The palms of the hands touch the temporo-buccal regions of the fetus. The head is first pulled down until one of the shoulders appears under the pubic arch.
Next, the head is taken with the left hand and lifted up, and the right crotch is shifted from the back shoulder, which is carefully removed. Having freed the shoulder part, lift the torso up by the armpits.
In some cases, to prevent intracranial injury, a perineotomy is performed if the perineum is unyielding.
Complications
Although occiput anterior births routinely show a biomechanism, complications can occur. The size of the pelvis strongly affects the possibility of a successful delivery. Difficult births occur if the woman in labor has a narrow pelvis. This pathology is quite rare. It serves as a reason for the decision to carry out a planned caesarean section. There are other adverse factors that can complicate childbirth: a large or overdue fetus. In these cases, a planned caesarean section is often chosen. In some cases, the need to end childbirth through a caesarean section appears only in their course.
