Fetal Skull Anatomy

Fetal Skull

Regions of the Fetal Skull

The fetal skull is divided into the following regions:

• Face
• Brow (sinciput)
• Vertex
• Occiput

Bones of the Vault

The bones of the vault include:

• Occipital bone: Located at the posterior part of the head, partly contributing to the base of the skull.
• Parietal bones: Two bones on either side of the skull. Once ossified, they form parietal eminences.
• Frontal bones: Two bones that shape the forehead/sinciput, ossifying to form the frontal eminence of each bone. By the age of eight, these two bones fuse into one.
• Temporal bone: Upper segment on both sides of the head, forming part of the vault's structure.

Ossification, Sutures, and Fontanelles

• Ossification: The natural process of bone formation/hardening.
• Incomplete Ossification at Birth: The process of ossification is incomplete at birth, resulting in sutures between the bones and fontanelles where two or more sutures meet.

Sutures and Moulding

• Lambdoid (or lambdoidal) suture: Separates the occipital bone from the two parietal bones.
• Sagittal suture: Lies between the two parietal bones.
• Coronal suture: Separates the two frontal bones from the parietal bones, passing from one temple to another.
• Frontal suture: Separates the two frontal bones.
• Moulding: This is the change in the shape of the fetal head during its passage through the pelvis and birth canal during labor/birth. The sutures enable moulding during birth.
• Alteration in shape is possible because the bones of the vault allow a slight degree of bending, and the skull bones can override at the sutures.
• Overriding: Allows a considerable reduction in the size of the presenting diameters.

Fontanelles

• Anterior (Bregma): Diamond or kite-shaped, closes at approximately 18 months of age.
• Posterior (Lambda): Small, triangular shape, closes at approximately 6 weeks of age.
• Together, the sutures and fontanelles allow for overlap of the fetal skull bones during moulding.

Clinical Insight

• Palpating the sagittal suture during a Vaginal Examination (VE) during labor provides insight into fetal head engagement (Asynclitic or synclitic), the degree of internal head rotation, and head moulding.

Importance of Understanding Fetal Skull

• Understanding the fetal skull and the bony pelvis is essential for assessing the position of the fetus and the descent of the presenting part through abdominal palpation and vaginal examination.
• Knowledge of fetal skull bones and dimensions is required for reviewing the maternal pelvic bones.
• The fetal head needs to fit through the maternal pelvis to successfully birth.
• The relationship between the fetal skull bones and the maternal pelvis will be examined further.