Fetal Skull Anatomy and Diameters

Bones and Sutures

• Frontal Bones: Two frontal bones marked with 'f' on the fetal skull. Separated by the frontal suture. These bones gradually fuse during early childhood. The prominence of the frontal bones and the metopic suture (frontal suture) can sometimes indicate developmental conditions.

• Parietal Bones: Two parietal bones forming the sides and roof of the cranium. They articulate with the frontal bone at the coronal suture, with each other at the sagittal suture, and with the occipital bone at the lambdoid suture.

• Sagittal Suture: Lies between the two parietal bones. It allows for the expansion of the brain during infancy and early childhood. Premature closure of the sagittal suture (sagittal synostosis) can lead to scaphocephaly, a long, narrow skull shape.

• Coronal Suture: Separates the frontal bones from the parietal bones. Unilateral or bilateral premature fusion of the coronal sutures can lead to plagiocephaly (asymmetrical skull shape) or brachycephaly (flattened forehead), respectively.

• Lambdoid Suture: Separates the occipital bone from the parietal bones. Premature fusion is less common but can result in skull deformities.

• Occipital Bone: Located at the back of the skull, it articulates with the parietal bones via the lambdoid suture. The foramen magnum, the large opening in the occipital bone, allows passage of the spinal cord.

Fontanelles

• Definition: Gaps between the suture lines, filled with fibrous membrane. These gaps allow for skull molding during birth and brain growth after birth.

• Number: Fetal skull has six fontanelles at birth: anterior, posterior, two mastoid (posterolateral), and two sphenoidal (anterolateral) fontanelles.

• Most Important Fontanelles: Anterior and posterior fontanelles due to their size and clinical significance in assessing infant development and hydration status.

Anterior Fontanelle (Bragma)

• Shape: Diamond-shaped or rhomboid-shaped.

• Boundaries:

  • Anteriorly: Frontal suture.

  • Posteriorly: Sagittal suture.

  • Laterally: Left and right coronal sutures.

• Diameters:

  • AP diameter: $3$ cm (approximately). Clinically, it ranges from 2 to 4 cm.

  • Transverse diameter: $3$ cm (approximately). Similar range as AP diameter.

• Ossification: Typically closes by $18$ months after birth. Delayed closure can indicate conditions such as hydrocephalus, hypothyroidism, or rickets.

• Clinical Significance: Palpation provides valuable information about intracranial pressure and hydration status. A bulging fontanelle may indicate increased intracranial pressure (e.g., meningitis, hydrocephalus), while a sunken fontanelle may suggest dehydration.

Posterior Fontanelle (Lambda)

• Shape: Triangular-shaped.

• Boundaries:

  • Anteriorly: Sagittal suture.

  • Laterally: Left and right lambdoid sutures.

• Location: Lies close to the occiput (occipital bone).

• Ossification: Typically ossifies by $2$ to $3$ months after birth. Earlier closure compared to the anterior fontanelle.

• Clinical Significance: Its closure is a marker of early skeletal development. Abnormalities can suggest underlying bone disorders.

Occiput and Synciput

• Occiput: Bony prominence on the occipital bone (occipital protuberance). It is an important landmark in determining fetal head position during labor.

• Synciput: Part of the skull anterior to the anterior fontanelle, comprising the frontal bones. Rounded part felt upon palpation during vaginal examination to assess fetal presentation.

Diameters of the Fetal Skull

General Rule

• AP diameters are generally longer than transverse diameters, influencing the way the fetal head engages and descends through the birth canal.

AP Diameters

• Longest AP Diameter: Mento-vertical diameter ($14$ cm). Extends from the mentum (chin) to the vertex. Significant in brow presentation.

• Second Longest AP Diameters:

  • Submento-vertical ($11.5$ cm). From the mentum to the highest point on the sagittal suture.

  • Occipito-frontal ($11.5$ cm). From the occipital protuberance to the fronto-nasal junction.

Clinical Significance: Brow Presentation

• In brow presentation, the engaging AP diameter is mento-vertical ($14$ cm), which is the longest diameter.

• A fully dilated cervix is $10$ cm. The inability of the fetal head to negotiate the birth canal with such a large diameter necessitates a Cesarean section.

• Management: Persistent brow presentation usually requires Cesarean delivery to avoid obstructed labor and associated complications.

Transverse Diameters

• Always smaller than AP diameters. These are critical in determining whether the fetal head can pass through the pelvic inlet and midpelvis.

• Mnemonic: Miss Tina So Pretty

  • M: Bimastoid diameter ($7.5$ cm). Measured between the mastoid processes.

  • T: Bitemporal diameter ($8$ cm). Measured between the temporal bones.

  • S: Subparietal diameter ($8.5$ cm). Measurement from the parietal bone just below the parietal eminence on one side to a similar point on the opposite side.

  • P: Biparietal diameter ($9.5$ cm). The largest transverse diameter, measured between the parietal eminences.

• Largest Transverse Diameter: Biparietal diameter ($9.5$ cm). Crucial for engagement in vertex presentations.

Application to Questions

• Smallest Diameter: Look at transverse diameters only, typically the bimastoid diameter.

• Longest Diameter: Look at AP diameters only, most often the mento-vertical diameter.

Parts of the Fetal Skull

Vertex

• Definition: Part of the skull between the anterior and posterior fontanelles, bounded laterally by the parietal eminences.

• Seen In: Fully flexed or partially flexed head, which is the most common presentation.

• Engaging Diameter (Vertex Presentation): Suboccipito-bragmatic ($9.5$ cm); allows for the smallest diameter to pass through the pelvis efficiently. The head appears to be crying ('sob').

Brow

• Definition: Part of the skull between the anterior fontanelle and the root of the nose.

• Seen In: Partially extended head, less common and often unstable.

• Engaging Diameter (Brow Presentation): Mento-vertical ($14$ cm), making vaginal delivery unlikely and usually requiring Cesarean section.

• Clinical Implications: Brow presentation is often transient, converting to either vertex or face presentation during labor. However, persistent brow presentation necessitates careful management.

Face

• Definition: Part of the skull between the root of the nose and the chin.

• Seen In: Fully extended head.

• Engaging Diameter (Face Presentation):

  • Submento-bragmatic ($9.5$ cm). From below the chin to the anterior fontanelle.

  • Submento-vertical ($11.5$ cm). From below the chin to the highest point on the sagittal suture.

• Management: Vaginal delivery may be possible with a mento-anterior (chin anterior) position. Mento-posterior (chin posterior) positions often require Cesarean delivery.

Important Diameters Measuring 9.5 cm

• Suboccipito-bragmatic (vertex presentation fully flexed).

• Submento-bragmatic (face presentation).

• Biparietal diameter (transverse diameter).

Deflexed Head

• Presentation is vertex but with the head less flexed than optimal.

• Engaging diameter is occipito-frontal ($11.5$ cm) or suboccipito-frontal ($10$ to $10.5$ cm).

• Engagement is delayed as engaging diameters are larger than suboccipito-bragmatic, potentially leading to prolonged labor.

Locations of Specific Diameters

• Suboccipito-bragmatic: From below the occipital protuberance (nape of the neck) to bragma; seen in vertex presentation with fully flexed head. This is the optimal diameter for vaginal delivery.

• Suboccipito-frontal: From nape of the neck to the mid of the frontal bone; seen in deflexed head.

• Occipito-frontal: From occipital protuberance to the root of the nasal bone; seen in deflexed head. Larger diameter delays engagement.

• Mento-vertical: From the chin to the highest point on the sagittal suture; longest diameter, seen in brow presentation, requiring Cesarean section.

• Submento-vertical: From below the chin to the highest point on the sagittal suture; seen in face presentation. Allows for vaginal delivery in mento-anterior positions.

• Submento-bragmatic: From below the chin to the bragma; seen in face presentation. Optimal diameter in face presentation.

Molding

• Definition: Alteration in the shape of the head as it passes through the birth canal during labor. Temporary and physiological adaptation.

• Grades:

  • Grade 0: No molding. Skull bones are not touching.

  • Grade 1: Skull bones touching each other but not overlapping. Minimal molding.

  • Grade 2: Skull bones overlap each other but can be separated. Moderate molding.

  • Grade 3: Skull bones overlap each other and cannot be separated (fixed overlap). Severe molding.

• Significance:

  • Grade 1 molding can be normal and is commonly observed.

  • Grades 2 and 3 indicate potential cephalopelvic disproportion (CPD), especially with slow progress of labor. Requires careful assessment.

• Clinical Assessment: Molding should be assessed in conjunction with other factors such as cervical dilatation, station, and maternal progress in labor.

Swellings on the Fetal Head

Caput Succedaneum

• Definition: Edematous swelling above the periosteum involving subcutaneous tissue. Occurs due to pressure against the cervix.

• Cause: Prolonged stagnation of the head in one position during labor, leading to impaired venous return and lymphatic drainage.

• Characteristics:

  • Pits on pressure (soft and compressible).

  • Can cross suture lines as it is superficial to the periosteum.

  • Present at birth; disappears within a few hours to days without intervention.

• Management: None required; resolves spontaneously. Parental reassurance is often sufficient.

• Association: If associated with slow progress of labor, indicates potential CPD and the need for further evaluation.

Cephal Hematoma

• Definition: Collection of blood below the periosteum. Limited by suture lines.

• Cause: Traumatic instrumental delivery (e.g., forceps, vacuum extraction) or prolonged labor.

• Characteristics:

  • Does not pit on pressure (firm and tense).

  • Cannot cross suture lines as it is confined by the periosteum.

  • Appears a few hours after birth; disappears in a few weeks to months.

• Management: None required in most cases; resolves automatically. Monitor for jaundice.

• Associations:

  • Can be associated with fracture of underlying bone due to trauma. Radiological evaluation may be necessary.

  • Can be associated with jaundice due to hemolysis and bilirubin production. Monitor bilirubin levels and manage appropriately.

Differentiation from Meningocele

• Meningocele:

  • Definition: A sac-like protrusion of the meninges through a defect in the skull.

  • Cough impulse present (may increase in size with coughing or crying).

  • Transillumination test positive (light passes through the sac).

• Cephal Hematoma:

  • Cough impulse absent.

  • Transillumination test negative.

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