Unlocking the Secrets of the Fetal Skull: Understanding Their Importance

▶ Areas of Fetal Skull
▶ Sutures
▶ Fontanels
▶ Diameter of Fetal Skull
▶ Circumference attitude of Fetal Skull
▶ Moulding
▶ Caput Succedaneum

The 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.

Fetal

Area of Fetal Skull

The skull is arbitrarily divided into several zones of obstetrical importance.

These are:

• Vertex

• Brow

• Face

Vertex
It is a quadrangular area bounded anteriorly by the bregma and coronal suture behind 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 the root of the nose and supraorbital ridges and on the other side by the junction of the floor of the mouth with the neck.

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

Structures of the Fetal skull

The sagittal or longitudinal suture lies between two parietal bones.
The coronal sutures run between parietal and frontal bones on either side.
The frontal suture lies between two frontal bones.
The lambdoid sutures separate the occipital bone and the two parietal bones.

Importance

It permits the gliding movement of one bone over the other during moulding of the head, a phenomenon of significance while the head passes through the pelvis during labor.

Digital palpation of the sagittal suture during internal examination in labor gives an idea of the manner of engagement of the head (asynclitism or syncretism), degree of internal rotation of the head, and degree of moulding of the head.

What are fontanels and their importance?

The wide gap in the suture line is called fontanel. Of the many fontanels (6 in number), two are of obstetrics significance:

• Anterior fontanel or bregma

• Posterior fontanel or lambda

Anterior fontanel
It is formed by joining the four sutures in the midplane. The sutures are anteriorly frontal, posteriorly sagittal, and on either side, coronal. The shape is like a diamond. Its anteroposterior and transverse diameters measure approximately 3 cm each. The floor is formed by a membrane and it becomes ossified 18 months after birth. It becomes pathological if it fails to ossify even after 24 months.
Posterior fontanel
It is formed by the junction of three suture lines- sagittal suture anteriorly and lambdoid suture on either side. It is triangular in shape and measures about 1.2 x 1.2 cm (1/2" x 1/2"). Its floor is membranous but becomes bony at term. Thus, truly its nomenclature as fontanel is a misnomer. It denotes the position of the head in relation to the 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-thirds and posterior one-third. It has got no clinical importance.

Importance

1. Its palpation through internal examination denotes the degree of flexion of the head.
2. It facilitates moulding of the head.
3. As it remains membranous long after birth, it helps in accommodating the marked brain growth; the brain becoming almost double its size during the first year of life.
4. Palpation of the floor reflects intracranial tension.
5. Collection of the blood and exchange transfusion, on rate occasion, can be performed through it via the superior longitudinal sinus.
6. Cerebrospinal fluid can be drawn, although rarely, through the angle of the anterior fontanel from the lateral ventricle.

Engaging diameter of the Fetal Skull

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

• Biparietal diameter
• Super-subparietal
• Bitemporal diameter
• Bimastoid diameter

Biparietal diameter- 9.5 cm (3,3/4")
It extends between two parietal eminences. Whatever may be the position of the head, this diameter nearly always engages.

Super-subparietal- 8.5 cm (3,1/2")
It extends from a point placed above the other parietal eminence to a point placed above the other parietal eminence of the opposite side.

Bitemporal diameter- 8 cm (3,1/4")
It is the distance between the anteroinferior ends of the coronal suture.

Bimastoid diametr- 7.5 cm (3")
It is the distance between the tips of the mastoid processes. The diameter is incompressible and it is impossible to reduce the length of the bimastoid diameter by obstetricle operation.

Circumferences of the Fetal head in different attitudes

The circumference of the plane of the diameter of engagement differs according to the attitude of the head.

Moulding of the Fetal Skull

It is the alteration of the shape of the forecoming head while passing through the resistant birth passage during labor. There is, however, very little alteration in the size of the head, as the volume of the content inside the skull is incompressible although a small amount of cerebrospinal fluid and blood escape out in the process. During normal delivery, an alteration of 4mm in skull diameter commonly occurs.

Mechanism

There is compression of the engaging diameter of the head with the corresponding elongation of the diameter at the right angle to it.
Thus, in the well-flexed head of the anterior vertex presentation, the engaging suboccipitobregmatic diameter is compressed with elongation of the head in mentoverticle diameter which is at the right angle to suboccipitobregmatic.

Grading

There are three gradings.

• Grade-1 - the bones touching but not overlapping

• Grade-2-  overlapping but easily separated 

• Grade-3- fixed overlapping

Importance of Moulding

Slight molding is inevitable and beneficial. It enables the head to pass more easily, through the birth canal.
The extreme moulding as met in disproportion may produce severe intracranial disturbance in the form of tearing of tentorium cerebelli or subdural hemorrhage.
The shape of the moulding can be a piece of useful information about the position of the head occupied in the pelvis.

Caput Succedaneum of the Fetal Skull

Caput succedaneum is the formation of swelling due to the stagnation of fluid in the layers of the scalp beneath the girdle of contact.

The girdle of contact is either bony or the dilating cervix or vulval ring. The swellings are diffuse, boggy and are not limited by the suture line. it may be confused with cephalhematoma. Caput disappears spontaneously within 24 hours after birth.

Mechanism of formation

While the head descends to press over the dilating cervix or vulval ring, the overlying scalp is free from pressure, but the tissues in contact with the full circumference of the girdle of contact are compressed. This interferes with venous return and lymphatic drainage from the unsupported area of the scalp - stagnation of fluid and appearance of swelling in the scalp. Caput usually occurs after rupture of the membranes.

Importance

It signifies the static position of the head for a long period of time.

The location of the caput gives an idea about the position of the head occupied in the pelvis and the degree of flexion achieved. In the left position, the caput is placed on the right parietal bone. With increasing flexion, the caput is placed more posteriorly.