Intrapartum diagnosis
The main method for detecting the pathology is a vaginal examination. It becomes informative only from the moment the head begins to engage in the pelvic inlet (in the first stage of labor, when cervical dilation reaches 2 cm or more).
What the doctor pays attention to:
- The sagittal suture is palpated in the transverse or slightly oblique pelvic diameter.
- Main diagnostic criterion: the large and small fontanelles are located strictly at the same level.
- If the sagittal suture is in the oblique diameter, the large fontanelle is always directed anteriorly.
- Quite often, the obstetrician determines slight asynclitism: the anterior parietal bone enters the pelvic inlet first, causing the sagittal suture to shift posteriorly (closer to the sacrum).
There is also a retrospective (postnatal) sign: if the delivery was vaginal, the newborn's head acquires a characteristic brachycephalic shape due to specific molding.
Biomechanism of labor: step-by-step analysis
The process of the fetus passing through the birth canal in sinciput presentation consists of five classical steps, but has its own biomechanical features.
- First step — moderate extension of the head. It occurs at the very beginning when the head just enters the pelvic inlet. The passing diameter becomes the occipitofrontal diameter (12 cm).
- Second step — internal rotation of the head. This stage begins in the wide part of the pelvic cavity. Rotation always occurs with the formation of the posterior position. The process is completed in the plane of the narrow part and pelvic outlet: the sagittal suture is finally established in the anteroposterior diameter.
- Third step — flexion of the head. A key stage where the points of fixation are formed. First, the first point of fixation is formed: the glabellar region of the fetus fits under the lower edge of the pubic symphysis. Flexion occurs, allowing the parietal eminences to be born. Immediately after this, the second point of fixation is formed: the fetal occiput rests against the mother's coccyx.
- Fourth step — extension of the head. Resting its occiput against the coccyx (second point of fixation), the head undergoes extension. As a result of this movement, the facial part is born from under the pubic symphysis.
- Fifth step — internal rotation of the shoulders and external rotation of the head. This final stage of the biomechanism proceeds in an absolutely identical manner to the classic occiput presentation.
Clinical course and obstetric tactics
The clinical picture of labor in first-degree extension has a number of dangerous features. Due to incorrect engagement, the head cannot form a proper girdle of contact. As a result, the amniotic fluid does not separate into forewaters and hindwaters, which often provokes premature rupture of membranes.
In addition, this pathology is characterized by a prolonged, exhausting course of both the first and second stages of labor. The prolonged stay of the fetus in the birth canal and passage of the large occipitofrontal diameter significantly increase the risk of hypoxia and mechanical trauma to the child.
Management of labor:
- A strictly individual approach to choosing the method of delivery is required in each specific case.
- The doctor is obliged to perform a thorough and repeated assessment of the ratio between the mother's pelvic dimensions and the fetal head.
- At the slightest doubt regarding fetopelvic disproportion, emergency cesarean section (CS) is indicated.
- Surgical delivery is especially relevant if, upon full cervical dilation, the obstetrician detects a tendency toward cephalopelvic disproportion.
Briefly: answers to topical questions
What is the circumference of the fetal head that crowns in a sinciput presentation?
In a sinciput presentation, the circumference of the fetal head corresponding to the occipitofrontal diameter crowns. The occipitofrontal diameter is 12 cm, and the corresponding circumference is 34–35 cm.
How does the vaginal finding in sinciput presentation differ from that in brow presentation?
The vaginal findings in sinciput and brow presentations differ by the palpated sutures and bony landmarks.
| Sign | Anterior vertex presentation | Brow presentation |
|---|---|---|
| Palpated suture | Sagittal suture (in transverse or slightly oblique diameter) | Frontal suture |
| Landmarks | Large and small fontanelles are at the same level | Bridge of the nose, supraorbital ridges, eye sockets, and large fontanelle |
In sinciput presentation, if the sagittal suture is located obliquely, the large fontanelle is anterior. In brow presentation, landmarks are determined by the ends of the frontal suture: the bridge of the nose and eye sockets are palpated on one side, and the large fontanelle on the other.
Where is the caput succedaneum localized on the newborn's head in sinciput presentation?
In sinciput presentation, the caput succedaneum is localized in the region of the large fontanelle.
The site of caput succedaneum formation always corresponds to the leading point — the lowest point of the presenting part that passes first along the axis of the birth canal. In sinciput presentation (I degree head extension), this leading point is the large fontanelle. The caput succedaneum itself is subcutaneous edema located superficially over the bone and periosteum. As a result of vaginal delivery, the head acquires a characteristic brachycephalic shape.
To memorize the sequence of fixation points in the third and fourth steps of the biomechanism, use the rule: "Flexed in front, extended in the back." First, the glabella is fixed anteriorly under the pubic bone (the head flexes, parietal eminences are born), then the occiput is fixed posteriorly near the coccyx (the head extends, the face is born).
Class and exam questions
How to determine sinciput presentation during a vaginal examination?
The main sign is that the large and small fontanelles are palpated at the same level. Meanwhile, the sagittal suture is in the transverse or slightly oblique diameter, and if oblique, the large fontanelle faces anteriorly. The examination is performed at a cervical dilation of 2 cm.
In what position do deliveries always occur in this pathology?
Internal rotation of the head in the wide part of the pelvic cavity always ends in the formation of the posterior position.
What fixation points are formed during the expulsion of the fetus?
First, the first point of fixation is formed — the glabellar region rests under the lower edge of the pubic symphysis (flexion occurs). Then the second point is formed — the occiput rests against the coccyx (extension and birth of the face occur).
Why is early rupture of membranes common in fronto-anterior presentation?
Due to extension, the head cannot tightly press against the pelvic bones and form the girdle of contact, which normally prevents premature rupture of amniotic fluid.
What else to study on this topic
- Brow presentation (II degree of head extension)
- Face presentation (III degree of head extension)
- Clinically narrow pelvis and signs of disproportion
- Asynclitic engagements of the fetal head
- Diagnosis and prevention of fetal hypoxia in prolonged labor
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