FETAL SKULL

Table of Contents

The skull bones encase and protect the brain, which is very delicate and subjected to pressure when the fetal head passes down the birth canal.

Fetal skull is to some extent compressible and made mainly of thin pliable tabular (flat) bones forming the vault. This is anchored to the rigid and incompressible bones at the base of the skull.

AREAS OF SKULL: The skull is arbitrarily divided into several zones of obstetrical importance
These are:

Vertex : It is a quadrangular area bounded anteriorly by the bregma and coronal sutures behind by the lambda and lambdoid sutures and laterally by lines passing through the parietal eminences.
Brow : It is an area bounded on one side by the anterior fontanel and coronal sutures and on the other side by the root of the nose and supraorbital ridges of either side.
Face : It is an area bounded on one side by root of the nose and supraorbital ridges and on the other, by the junction of the floor of the mouth with neck.

Fetal skull showing different regions and landmarks of obstetrical significance

Sinciput is the area lying in front of the anterior fontanel and corresponds to the area of brow and the occiput is limited to the occipital bone.
Flat bones of the vault are united together by non-ossified membranes attached to the margins of the bones. These are called sutures and fontanels. Of the many sutures and fontanels, the following are
of obstetric significance.

Bones of the Vault

The bony structure of the vault originates within a membrane framework. Over time, a process known as ossification hardens these structures from the center outward.

At birth, ossification remains incomplete, resulting in small gaps existing between the bones referred to as sutures and fontanelles. Each bone features a distinct ossification center, which appears as a noticeable protrusion. The full ossification of the skull takes place only in early adulthood.

The vault’s bony composition encompasses:

The occipital bone, located at the posterior of the head. A portion of this bone contributes to the skull’s base, encompassing the foramen magnum—a protective passage for the spinal cord as it exits the skull. The occipital protuberance marks the site of ossification.
The two parietal bones situated on either side of the skull. These bones’ ossification centers are termed parietal eminences.
The two frontal bones, shaping the forehead or sinciput. Ossification initiates at the frontal eminence of each bone. These frontal bones fuse into a singular entity by the age of eight.
The upper segment of the temporal bone on both sides of the head participates in forming the vault’s structure.

Regions and landmarks of the fetal skull

The fetal skull’s various segments are defined by distinct regions, each marked by significant landmarks(see figure above). These points of reference hold particular importance for midwives during vaginal examinations, aiding in determining the fetal head’s position.

• The occiput region occupies the space between the foramen magnum and the posterior fontanelle. The area below the occipital protuberance (landmark) is referred to as the sub-occipital region.

• The vertex region is enclosed by the posterior fontanelle, the paired parietal eminences, and the anterior fontanelle.

• The forehead, or sinciput region, spans from the anterior fontanelle and the coronal suture to the orbital ridges.

• Extending from the orbital ridges and the base of the nose to the junction of the chin, or mentum (landmark), and the neck is the face region. The point situated between the eyebrows is recognized as the glabella

SUTURES

The sagittal or longitudinal suture is situated between two parietal bones.
The coronal sutures run between the parietal and frontal bones on both sides.
The frontal suture is positioned between two frontal bones.
The lambdoid sutures separate the occipital bone and the two parietal bones.

Importance:

It allows smooth movement of one bone over the other during head molding, which is significant as the head passes through the pelvis during labor.
Palpating the sagittal suture during internal examination in labor provides insight into head engagement (asynclitism or synclitism), the degree of internal head rotation, and head molding.

FONTANELS

A wide gap in the suture line is referred to as a fontanel. Among the numerous fontanels (total of 6), two hold obstetric significance: (1) Anterior fontanel or bregma and (2) Posterior fontanel or lambda.

Anterior fontanel: It results from the fusion of four sutures in the midline. The sutures include the frontal suture anteriorly, the sagittal suture posteriorly, and the coronal sutures on either side. Its shape resembles a diamond, with anteroposterior and transverse diameters of approximately 3 cm each. The floor consists of a membrane, which undergoes ossification around 18 months after birth. If ossification does not occur even after 24 months, it becomes pathological.

Importance:

Palpating it during internal examination indicates the degree of head flexion.
It aids in head molding.
Due to its membranous nature persisting after birth, it accommodates significant brain growth, with the brain nearly doubling in size during the first year of life.
Palpation of the floor reflects intracranial conditions – depressed in dehydration, elevated in raised intracranial pressure.
In rare cases, blood collection and exchange transfusion can be performed through it, via the superior longitudinal sinus.
Although uncommon, cerebrospinal fluid can be drawn through the angle of the anterior fontanel from the lateral ventricle.

Posterior fontanel: It is formed by junction of three suture lines — sagittal suture anteriorly and lambdoid suture on either side. It is triangular in shape and measures about 1.2 × 1.2 cm (1/2″ × 1/2″).
Its floor is membranous but becomes bony at term. Thus, truly its nomenclature as fontanel is misnomer.
It denotes the position of the head in relation to maternal pelvis.
Sagittal fontanel: It is inconsistent in its presence. When present, it is situated on the sagittal suture at the junction of anterior two-third and posterior one-third. It has got no clinical importance.

DIAMETERS OF SKULL

The engaging diameter of the fetal skull depends on the degree of
flexion present. The anteroposterior diameters of the head which may engage are:

Presentation	Diameter	(cm)	Attitude of the Head
Vertex	Suboccipitobregmatic — extends from the nape of the neck to the center of the bregma	9.5	Complete flexion
Vertex	Suboccipito-frontal — extends from the nape of the neck to the anterior end of the anterior fontanel or center of the sinciput	10	Incomplete flexion
Vertex	Occupitofrontal — extends from the occipital eminence to the root of the nose (Glabella)	11.5	Marked deflexion
Brow	Mento-vertical — extends from the midpoint of the chin to the highest point on the sagittal suture	14	Partial extension
Face	Submentovertical — extends from junction of floor of the mouth and neck to the highest point on the sagittal suture	11.5	Incomplete extension
Face	Submentobregmatic — extends from junction of floor of the mouth and neck to the center of the bregma	9.5	Complete extension

Transverse diameters

The transverse diameters of the fetal skull;

There are also two transverse diameters,
• The biparietal diameter (9.5 cm) – the diameter between the two parietal eminences.
• The bitemporal diameter (8.2 cm) – the diameter between the two furthest points of the coronal suture at the temples.

Knowledge of the diameters of the trunk is also important for the birth of the shoulders and breech

Bisacromial diameter 12 cm: This is the distance between the acromion processes on the two shoulder blades and is the dimension that needs to pass through the maternal pelvis for the shoulders to be born. The articulation of the clavicles on the sternum allows forward movement of the shoulders, which may reduce the diameter slightly.
Bitrochanteric diameter 10 cm: This is measured between the greater trochanters of the femurs and is the presenting diameter in breech presentation.

Presenting diameters

Some presenting diameters are more favourable than others for easy passage through the maternal pelvis and this will depend on the attitude of the fetal head.

This term attitude is used to describe the degree of flexion or extension of the fetal head on the neck. The attitude of the head determines which diameters will present in labour and therefore influences the outcome.
The presenting diameters of the head are those that are at right-angles to the curve of Carus of the maternal pelvis.
There are always two: a longitudinal diameter and a transverse diameter. The presenting diameters determine the presentation of the fetal head, for which there are three:

Vertex Presentation: When the head displays pronounced flexion, the sub-occipitobregmatic diameter (9.5 cm) and the biparietal diameter (9.5 cm) come into play. Given their equal length, the presenting area takes on a circular form, optimally conducive to cervix dilation and successful head birth. The sub-occipitofrontal diameter (10 cm) is the dimension that expands the vaginal orifice. Conversely, when the head is deflexed, the presenting diameters shift to the occipitofrontal (11.5 cm) and the biparietal (9.5 cm). This circumstance often arises when the occiput occupies a posterior position. In such cases, if the posterior position persists, the diameter expanding the vaginal orifice will be the occipitofrontal (11.5 cm).
Face Presentation: Complete extension of the head leads to the submentobregmatic diameter (9.5 cm) and the bitemporal diameter (8.2 cm) serving as the presenting dimensions. The sub-mentovertical diameter (11.5 cm) is the dimension that stretches the vaginal orifice.
Brow Presentation: Partial extension of the head results in the mentovertical diameter (13.5 cm) and the bitemporal diameter (8.2 cm) becoming the presenting diameters. In instances where this presentation persists, vaginal birth becomes less likely.

Moulding

The term moulding is used to describe the change in shape of the fetal head that takes place during its passage through the birth canal.

Alteration in shape is possible because the bones of the vault allow a slight degree of bending and the skull bones are able to override at the sutures. This overriding allows a considerable reduction in the size of the presenting diameters, while the diameter at right-angles to them is able to lengthen owing to the give of the skull bones(Fig. 7.13).

The shortening of the fetal head diameters may be by as much as 1.25 cm. The dotted lines in Figs 7.14–7.19 illustrate moulding in the various presentations.
Additionally, moulding is a protective mechanism and prevents the fetal brain from being compressed as long as it is not excessive, too rapid or in an unfavourable direction. The skull of the pre-term infant is softer and
has wider sutures than that of the term baby, and hence may mould excessively should labour occur prior to term.

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