The Fetal Skull

The fetal head is the most significant component of the fetus because it is the most common presenting part: it is the largest and least compressible section of the fetus, and once born, it generally facilitates a smooth delivery of the rest of the body.

The fetal skull is built on big ossified, securely linked bones that are not easily compressed. Their functions are to protect the brain stem's essential centers. The vault is made up of somewhat compressible bones that can overlap at the suture lines. The following heading will be used to describe the skull:

• Areas of skull
• Vault of skull
• Suture lines
• Fontanelle
• Landmarks
• Diameters
• Miscellaneous

Areas of Skull

• Vertex: It is bordered anteriorly by the bregma and coronal sutures, posteriorly by the lambda and lambdoid sutures, and laterally by the line passing across the parietal eminences.
• Brow: It is bounded on one side by the anterior fontanelle and the coronal sutures, and on the other by the root of the nose and either side's supra-orbital ridges.
• Face: It is defined by the junction of the floor of the mouth and the neck on one side and the root of the nose and the supra-orbital ridges on the other.

Vault of Skull

Several bones, primarily the occiput posteriorly, the two parietal bones on either side and the two frontal and temporal bones anteriorly, contribute to its creation. Because these bones are created in a membrane, they are generally thin, poorly ossified, and compressible at the connections. The bone's loose fit at the suture lines allows for overlapping of the skull bone edge during molding, which helps the fetal skull conform to the contour of the maternal pelvis during childbirth. The cranium is wider posteriorly than anteriorly (biparietal diameter) (bitemporal diameter).

Sutures

• Sagittal suture: This connects two parietal bones.
• Coronal suture: This is located on either side between the frontal and parietal bones.
• Frontal suture: This connects two frontal bones.
• Lambdoid suture: This connects the parietal and occipital bones on either side.

Clinical Importance of Suture:

• These sutures allow gliding movement of one bone over the other during vertex presentation of the head, resulting in a reduced diameter of the skull and easier transit of the head through the birth canal.
• The fontanelle and sagittal suture positions can be used to determine the vertex's attitude and position.
• During labor, a degree of internal rotation and degree of head molding can be detected by digital palpation of the sagittal suture.
• This sagittal suture lies transversely at the level of the ischial in profound transverse arrest.

Fontanelle

Fontanelle is the membrane-filled gaps at the intersection of the suture lines. The anterior and posterior fontanelles are significant to the midwife since their detection aids in determining the location of the fetal health in the pelvis during labor. The anterior fontanelle is an important landmark in early neonatal life for determining the newborn's condition.

1. Anterior fontanelle or bregma:
   It is a diamond-shaped unossified membrane area generated by the intersection of four sutures. The sutures are as follows: Frontal suture - anterior Sagittal suture - posteriorly on both sides, lateral. It is felt like a gentle shallow dip on the fetal head surface. By 18 months after birth, it has ossified.
    
2. Posterior fontanelle or lambda:
   It is the triangle depressed area formed by the three sutures. Sagittal suture - anterior Posteriorly, there are two lambdoid sutures on each side. 

Landmarks

1. Occiput: A portion of the occiput bone that is located behind the posterior fontanelle.
2. Sinciput: Area delimited inferiorly by the glabella and orbital ridges and superiorly by the bregma and coronal sutures.
3. Vertex: The area between the anterior and posterior fontanelles, as well as the two parietal eminences laterally. It is a representation of the top of a skull.
4. Bregma: The enormous diamond-shaped anterior fontanelle.
5. Glabella: The raised area between the orbital ridges.
6. Nasion: It is the root of the nose.
7. Parietal eminences: The parietal bones' basal region. The biparietal diameter is the distance between the two eminences.

Diameter of Skull

The fetal skull's engaging diameter is determined by the degree of bending of the presenting part.

Anteroposterior 
It runs from the nape of the neck to the anterior fontanelle.
Length:- 9.5cm
Attitude: Complete flexion
Presentation: Vertex 
Clinical Importance: Smallest diameter.

1. Suboccipital frontal
   It runs from the nape of the neck to the tip of the nose.
   Length: 10cm
   Attitude: Incomplete flexion.
   Presentation: Vertex.
2. Occipito-frontal
   The root of the nose extends from the occipital eminence (Glabella).
   Length: 11.5cm
   Attitude: Marked deflexion
   Presentation: Vertex
   Clinical importance: This engaging diameter may give rise to prolonged labor.
3. Mento-vertical
   It runs from the chin's midpoint to the sagittal suture's center.
   Length: 14cm
   Attitude: Partial extension.
   Presentation: Brow
   Clinical importance- In this engaging diameter, a baby has to be delivered by cesarean section.
4. Sub-mento vertical
   It runs from the junction of the mouth floor and the neck.
   Length: 11.5cm
   Attitude: Incomplete extension.
   Presentation: Face
   Clinical importance: In this engaging diameter, a baby has to be delivered by cesarean section.
5. Sub-mento bregmatic
   It stretches from the intersection of the mouth's floor and neck.
   Length: 9.5cm
   Attitude : Complete extension
   Presentation: Face
   Clinical importance: In this engaging diameter, the baby has to be delivered by cesarean section

Transverse diameter

1. Biparietal diameter
   It stretches between two parietal eminences.
   Length: 9.5cm
   Attitude: Irrespective of a position of a head this diameter always engages.
2. Bitemporal diameter
   The distance between the coronal suture's anterior and inferior ends.
   Length:- 8.5cm.

Miscellaneous

Molding

Molding refers to the fetal head's capacity to change shape and so adapt to the unyielding maternal pelvis. Suture lines allow the skill bone to slip over one another. Because the sacral promontory exerts significant pressure on the posterior parietal bone, it passes beneath the anterior parietal bones, allowing for midline overlapping of the parietal bones. The occipital and frontal bones move beneath the parietal bones. Molding aids in the reduction of presenting diameters; however, the skull experiences a corresponding increase in diameter at right angles to it, ensuring that the volume of the cranium is not reduced and the brain is not harmed.

Excessive molding in vertex presentation can result in mento-vertical diameter elongation, tearing of the tentorium cerebelli, and rupture of the vein of Galen, culminating in severe cerebral hemorrhage.

Grading of Molding

1. ‘0’ - No molding
2. ‘1’ - The bones touching but not overlapping
3. ‘2’ - Overlapping but easily separated
4. ‘3’ - Fixed overlapping

References

• Nursing Crib. 11 November 2010. http://nursingcrib.com/anatomy-and-physiology/the-fetal-skull/
• GFMER. 22 September 2016 http://www.gfmer.ch/Obstetrics_simplified/foetal_skull.htm
• Quizlet. https://quizlet.com/8585093/pelvis-the-fetal-skull-105-flash-cards
• Sweet, B.r., 1989, Maye's midwifery, A textbook for midwives; 11th edition, Bailliere Tindall, London, Philadelphia
• Tuitui R. 2002, A textbook of Midwifery A (Antenatal), 3rd edition, Vidyarthi Pustak Bhandari (Publisher and Distributor), Bhotahity, Kathmandu