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Anteroposterior, frontal and facial presentation of the fetus
Last reviewed: 05.07.2025
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Causes of extension presentation of the fetus
The reasons for the formation lie in the characteristics of the organisms of the pregnant woman and the fetus, due to which the presenting part of the fetus cannot be correctly positioned above the entrance to the small pelvis.
Maternal causes include overstretching of the uterus due to polyhydramnios, multiple pregnancy, multiple births, abnormal shape of the uterus - saddle-shaped, bicornuate, presence of a septum in the cavity, fibroids.
Fetal causes include small or too large sizes of the fetal head (prematurity, anencephaly, microcephaly, hydrocephalus), the presence of cervical teratomas, and thyroid tumors.
An important reason for the extension insertion of the head is the clinical discrepancy between the sizes of the head and the pelvis, in particular with narrow pelvises, large fetuses, and tumors of the soft and bone tissues of the small pelvis.
The biomechanism of labor in all types of extension presentation has common features: in all variants of extension presentation, labor is possible only if the posterior view is formed, which is especially important for labor in the face presentation.
Cephalic presentation
Anterior cephalic presentation is often found with a flat pelvis, i.e. with reduced direct dimensions of the pelvic planes with normal transverse ones. In such cases, the head remains above the entrance to the small pelvis for a long time with a sagittal suture in the transverse dimension, both fontanelles are at the same level. If, as a result of adaptive movements, the fetal head straightens (the first moment of the biomechanism of labor), then the large fontanelle will become the leading point and will be the first to descend into the small pelvis. The large segment of the head in this case will correspond to the circumference in its direct dimension (12 cm). The birth tumor is formed in the area of the large fontanelle, the head acquires a tower-shaped configuration.
When passing from the wide to the narrow part of the small pelvis, the head begins an internal rotation (the second moment of the biomechanism of labor), completed in the plane of exit from the small pelvis with the formation of the posterior view. The identification lines on the presenting part will be the sagittal suture and part of the frontal suture.
The first fixation point is formed on the presenting part - between the bridge of the nose and the lower edge of the pubic bone. The occipital part of the head, under the action of expelling forces directed along the axis of the spine, continues its forward movement. This determines the third moment of the biomechanism of labor - flexion of the head. Clinically, this moment corresponds to the birth of the large fontanel and parietal tubercles. The eruption of the head with a straight size and a circumference of 34 cm is often accompanied by trauma to the head and soft tissues of the birth canal.
After the head passes the plane of the small pelvis exit, the bridge of the nose slips out from under the pubis, and the occipital part of the head is fixed at the apex of the coccyx or sacrococcygeal joint, forming the second point of fixation with the suboccipital fossa. The fourth moment of the biomechanism of labor begins - extension of the head, which clinically corresponds to the birth of the fetus's face from under the pubis. The fifth moment of the biomechanism of labor - internal rotation of the shoulder girdle - does not differ from that in the occipital presentation.
The course of labor in the case of a head-anterior presentation, even in the case of normal fetal and pelvic sizes, is prolonged and requires a significant configuration of the head and vigorous labor.
For the diagnosis of anterior cephalic insertion during labor, external obstetric examination techniques are of little information, although the degree of extension of the head is sometimes determined using Leopold's 3rd and 4th techniques.
With sufficient dilation of the cervix and the absence of the amniotic sac, the internal obstetric examination has the greatest diagnostic value. The basis for diagnosing the anterior cephalic presentation (insertion) is the location of the large fontanelle on the leading axis of the pelvis and the sagittal suture, which is easily accessible for palpation.
In women with normal fetal and pelvic dimensions, uncomplicated obstetric history and regular labor, labor in the anterior cephalic presentation is carried out expectantly through the natural birth canal. In case of a complicated obstetric history and the slightest deviations from the normal course of labor, delivery by cesarean section is indicated.
Frontal presentation
A significant danger in labor is the frontal presentation. It is formed as a transition from the anterior cephalic to the facial. Spontaneous labor is possible extremely rarely for premature babies with low body weight or a dead fetus with autolysis.
The reasons for the frontal insertion are similar to those for other extension insertions. The large segment corresponds to the large oblique size of the head (13.5 cm, 39-41 cm in circumference).
The first moment of the biomechanism of labor is also the extension of the head. The leading point is the middle of the frontal suture, the first to enter the plane of the entrance to the small pelvis. A birth tumor is formed on the suture, and the head acquires a pyramidal shape.
The second moment of the biomechanism of labor - internal rotation of the head - also ends on the pelvic floor with the formation of the posterior view. The first fixation point is formed between the upper jaw of the fetus and the lower edge of the pubis. The third moment of the biomechanism of labor - flexion of the head - is performed. The birth of the head is similar to that described in the anterior non-cephalic presentation with a similar second fixation point and fourth moment of the biomechanism of labor. The shoulder girdle is born as in the occipital presentation®,
Timely diagnosis of brow presentation is of utmost importance, since even with normal pelvic dimensions, the birth of a fetus alive through the natural birth canal is impossible: the large oblique size of the head, by which insertion occurs, exceeds any other size in the small pelvis. Therefore, in order to avoid maternal trauma in cases of brow presentation, emergency delivery by cesarean section is necessary. In case of fetal death, the birth is completed by a feticide operation.
The diagnosis of brow presentation is based on data from external and internal obstetric examination, auscultation and ultrasound fetoscopy.
During external obstetric examination, Leopold's 3rd and 4th maneuvers allow one to determine the chin as a sharp protruding part on the head, and on the opposite side - a depression between the fetus's back and its occiput. The fetus's heartbeat will be better heard from the chest side.
Internal obstetric examination allows palpation of the frontal suture, brow ridges, bridge of the nose and bridge of the nose of the fetus.
Facial presentation
The most favorable type is the face presentation, since the vertical size of the head, corresponding to the large segment in the face presentation, is proportionate to the small oblique size of the fetal head - 9.5 cm. The leading point is the chin. This presentation is recognized by the vertical line of the face, when it becomes accessible to palpation.
The biomechanism of childbirth in the face presentation mirrors the biomechanism of the occipital presentation. The first moment - extension of the head - begins above the entrance to the small pelvis, reaches its maximum on the pelvic floor, as a result of which the leading point becomes the chin of the fetus. Internal rotation (the second moment) ends on the pelvic floor with the formation of the posterior view (along the back).
In the case of a posterior rotation, a fixation point is formed between the lower edge of the pubis and the hyoid bone, around which the head is bent - the third moment of the biomechanism of labor. It is necessary to take into account the high frequency of perineal tissue injuries due to the eruption of the head with a size close to the large oblique. The fourth moment of the biomechanism of labor in face presentation - internal rotation of the shoulders and external rotation of the head - occurs as in all head presentations.
Diagnosis of face presentation is based on data from external and internal obstetric examinations, and ultrasound data. X-ray examination has not lost its importance.
Differential diagnostics of face and pure breech presentation is extremely important. In face presentation, the height of the fundus of the uterus corresponds to the gestational age, in breech presentations it is somewhat higher. In the fundus of the uterus in face presentation, a large, loose part is found, in breech presentations - a round, dense, balloting head. Above the entrance to the small pelvis in face presentation, the chin and occiput of the fetus are palpated.
During an internal obstetric examination in the case of a face presentation, the chin and facial line are determined. Palpation of the jaws and palate complements the diagnosis. In the case of a breech presentation, the tip of the coccyx and the intergluteal fold are found. It is not recommended to insert a finger into the anus due to the high probability of injury to the perineum of the fetus.
Natural delivery is possible only if the posterior view is formed. Labor management is expectant; if the slightest complications occur, for example, premature rupture of the fetal bladder, weakness of labor, a cesarean section is performed. The formation of the anterior view is unacceptable, it requires urgent delivery by cesarean section, since with the head extended on the pelvic floor in the anterior view, further forward movement and extension are impossible (the head is already extended as much as possible!) and threaten the death of the fetus and rupture of the uterus.
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