Classification and dimensions
The type of presentation directly depends on the degree of extension of the fetal cervical spine. Each form is characterized by its own leading point (the part that descends into the pelvis first) and a specific dimension with which the head overcomes the planes of the birth canal.
| Type of presentation | Leading point | Engaging dimension | Diameter | Circumference |
|---|---|---|---|---|
| Sinciput (anterior asynclitism) | Anterior fontanelle | Straight (occipitofrontal) | 12 cm | 34–35 cm |
| Brow | Glabella | Large oblique | 13.5 cm | 41–42 cm |
| Face | Chin | Vertical | 9.5 cm | 32 cm |
The more extended the head is, the more the dimensions of the presenting part change, which critically affects the prognosis of labor.
Causes and developmental dynamics
Extensor presentations can form long before labor as well as directly during cervical dilation.
During pregnancy, this feature has no major practical significance. Before regular labor activity begins, the fetus is mobile, and the presentation often spontaneously corrects to the physiological flexed one.
However, there are persistent extensor presentations that do not resolve over time. They occur when there are voluminous mechanical obstacles:
- Fetal factors: extensive cervical cystic hygroma or large goiter preventing the chin from pressing against the chest.
- Maternal factors: large submucosal uterine myomas deforming the uterine cavity.
During labor, the position of the head is unstable. Variants can transition into one another:
- If flexion occurs during a sinciput presentation, it transforms into a normal occipital presentation.
- If extension increases during a brow presentation, it converts into a face presentation.
Dangers of brow presentation
Among all variants, brow presentation is considered the most unfavorable in obstetric practice.
In this pathology, the head attempts to pass through the bony pelvic ring with its absolute maximum — the large oblique dimension (diameter 13.5 cm, circumference 41–42 cm). This creates insurmountable spatial disproportion. Labor is accompanied by significant difficulties even with small fetal dimensions. Vaginal delivery in a brow presentation is possible only with a very low fetal birth weight.
General principles of the mechanism of labor
Despite differences in leading points and dimensions, the biomechanism of labor in all extensor presentations obeys general laws:
- In the plane of the pelvic inlet, primary extension of the head occurs.
- In the plane of the wide part of the pelvis, internal rotation of the head begins.
- Rotation always ends with the formation of the posterior position.
The formation of the posterior position is a critical condition for the completion of the biomechanism. Delivery in the anterior position with extensor presentations is physically impossible.
Briefly: answers to topical questions
What fixation points (hypomochlions) are formed during the crowning of the head in a sinciput presentation?
During the crowning of the head in a sinciput presentation, two fixation points are formed sequentially.
- First fixation point — the glabella area, which fits under the lower edge of the pubic symphysis (flexion occurs around it and the parietal eminences are born).
- Second fixation point — the occiput, which abuts against the coccyx (extension occurs around it).
What is the specific shape of head molding and where is the caput succedaneum located in a brow presentation?
In a brow presentation, a pronounced caput succedaneum is localized strictly in the forehead region (on the frontal bones). The specific name of the head molding shape is not provided in the supplied materials, however, it is indicated that the diagnosis of brow presentation is retrospectively confirmed precisely by the characteristic shape of the born child's head.
What specific steps does the complete biomechanism of labor include in a face presentation?
The complete biomechanism of labor in a face presentation includes four main steps.
- First step — maximum extension of the head (the occiput is maximally approximated to the fetal back).
- Second step — internal rotation of the head (the chin, or mentum, rotates anteriorly, forming the posterior position).
- Third step — flexion of the head (occurs around the fixation point — the hyoid bone beneath the pubic symphysis, or symphysis pubica).
- Fourth step — internal rotation of the shoulders and external rotation of the head.
By what anatomical landmarks during internal vaginal examination is a face presentation diagnosed?
During internal vaginal examination, face presentation is diagnosed by palpation of the fetal facial parts. The following anatomical landmarks are determined:
- Chin
- Mouth
- Nose
- Superciliary arches
During the examination, careful differential diagnosis with frank breech presentation is required so as not to mistakenly take the anus for the mouth, and the coccyx and ischial tuberosities for facial landmarks.
What is the obstetric management of delivery in a brow presentation of a term fetus of normal size?
In brow presentation, with normal dimensions of the pelvis and fetus, vaginal delivery is impossible. Cesarean section is indicated. Delay in operative delivery may lead to uterine rupture due to the development of a clinically narrow pelvis. Vaginal delivery is possible only in the persistent occipitoposterior (mentoposterior) position with very small fetal dimensions and a large pelvis, or if the brow presentation converts to a face presentation.
The degree of extension is easy to remember by mentally moving down the fetal face from top to bottom: Fontanelle (sincipital) → Bridge of the nose (brow) → Chin (face).
Class and exam questions
Is an extensor presentation detected on ultrasound in the third trimester clinically significant?
No, during pregnancy this has no practical significance. Prior to the onset of labor, presentation is unstable and can resolve spontaneously.
Why is brow presentation considered the most difficult?
Because the head engages in the pelvis with its largest dimension — the maximum transverse diameter (circumference 41–42 cm). This makes spontaneous delivery impossible, except in cases of extremely low fetal weight.
In which variety do deliveries occur in extensor presentations?
Exclusively in the posterior variety. Internal rotation in the wide part of the pelvic cavity always leads to the formation of the posterior variety, as delivery in the anterior variety with this position is impossible.
What else to study on this topic
- Detailed biomechanism of labor in sincipital presentation
- Points of fixation and stages of labor in face presentation
- Causes of the impossibility of labor in the anterior variety during head extension
- Differential diagnosis of presentation types during digital vaginal examination
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