Brow presentation

General characteristics and clinical significance

In obstetrical practice, brow presentation is extremely rare. This condition is characterized by the second degree of head extension. Often, this position is not final but serves merely as a transitional state: during labor, the presentation may transform from a sinciput into a face presentation.

The main obstetrical problem is that the large segment of the fetal head passes through the planes of the minor pelvis with its maximum diameter — the large oblique diameter, which is 13.5 cm. This is the largest dimension compared to all other extension insertion variants, making vaginal delivery extremely traumatic.

Diagnostic methods

Making an accurate diagnosis before the onset of labor presents significant difficulties.

• External maneuvers: diagnosis using them is extremely difficult. The physician can rely only on indirect signs suggesting pathology. These include a high standing of the uterine fundus and a palpable angle between the fetal occiput and back.
• Auscultation: due to extension of the fetal spine, its chest lies closer to the uterine wall. As a result, the heartbeat is auscultated more clearly on the side of the chest rather than the back.
• Vaginal examination: this is the key diagnostic method. During the examination, the physician identifies the frontal part of the head and palpates the frontal suture. The most important landmarks are the ends of this suture: on one side, the root of the nose, supraorbital ridges, and eye sockets of the fetus are palpated, and on the opposite side, the large fontanelle.

Biomechanism of labor

If labor is conducted via the natural birth canal, the biomechanism includes five sequential steps:

1. First movement — head extension. The process begins in the plane of the pelvic inlet. The frontal suture is established in the transverse diameter. The leading point is the frontal bones (a caput succedaneum forms over them during labor).
2. Second movement — internal rotation of the head. Rotation begins as the head descends deeper into the pelvis and ends in the plane of the pelvic outlet. The fetus turns with its back posteriorly, resulting in the frontal suture shifting into the direct pelvic diameter.
3. Third movement — head flexion. The first point of fixation is formed: the fetal upper jaw is pressed against the lower edge of the pubic symphysis. Flexion of the head occurs around this point, through which the vertex and occiput are born.
4. Fourth movement — head extension. The second point of fixation arises: the suboccipital fossa of the fetus approaches the tip of the coccyx. Extension of the head occurs around this second point, after which it is born completely.
5. Fifth movement — internal rotation of the shoulders and external rotation of the head. This final stage proceeds according to the same biomechanisms as in other variants of cephalic presentation.

Head molding and operative delivery

The diagnosis of brow presentation is often confirmed retrospectively — after the birth of the child, when assessing the shape of their head. Due to the specific configuration of the skull bones and the caput succedaneum localized strictly in the frontal region, the newborn's head acquires a characteristic "pyramid" or "tower" shape.

Due to the high risk of complications during labor, surgical interventions are frequently required:

• Cesarean section (CS): indicated in post-term pregnancy, as well as in cases where there is no progression of the head through the birth canal or failure to form the anterior type.
• Vacuum extraction of the fetus: the method is used in the development of uterine inertia or signs of fetal hypoxia. An important condition: the head must be located in the narrow part of the pelvic cavity.
• Episiotomy: indicated for similar complications (fetal hypoxia, uterine inertia) if the head is at the pelvic outlet.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Differential diagnosis with sinciput and face presentations.
• Features of vaginal examination in various degrees of head extension.
• Technique of vacuum extraction when the head is in the narrow part of the pelvic cavity.
• Indications and technique for performing episiotomy during labor.

Other topics in the "Obstetrics" section

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