Fetal Skull Flashcards

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

• Occipital Bone: Located at the posterior part of the head. It partly contributes to the base of the skull.
• Parietal Bones: There are two parietal bones, one on either side of the skull.
  • Once ossified, they form parietal eminences.
• Frontal Bones: There are two frontal bones that shape the forehead/sinciput.
  • They ossify to form the frontal eminence of each bone.
  • By eight years of age, the two bones fuse to become one.
• Temporal Bone: The upper segment of the temporal bone is on both sides of the head and part of the vault's structure.

Sutures

• Sutures remain between the bones due to the incompleteness of ossification.
• Sutures and moulding during birth:
  • 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
  • These sutures enable moulding during birth.

Moulding

• Moulding describes the change in shape of the fetal head that takes place during its passage through the pelvis and birth canal during labour/birth.
• Alteration in shape is possible because:
  • The bones of the vault allow a slight degree of bending.
  • The skull bones can override at the sutures.
  • This overriding allows a considerable reduction in the size of the presenting diameters (Marshall & Raynor, 2020).

Clinical Significance of Sutures

• Palpating the sagittal suture during a Vaginal examination (VE) during labour provides insight into:
  • Fetal head engagement (Asynclitic or synclitic).
  • The degree of internal head rotation.
  • Head moulding.

Fontanelles

• The fetal skull has two 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 the process of moulding.

Clinical Importance

• Understanding of the fetal skull and the bony pelvis is invaluable.
• It forms the basis for understanding assessment of:
  • The position of the fetus.
  • Descent of the presenting part by abdominal palpation and vaginal examination.

Fetal Skull Diameters

• Refer to Marshall, J.E. & Raynor, M.D. (2020) Myles Textbook for Midwives 17th ed. Elsevier for reading.

Fetal Skull and Maternal Pelvis

• Understanding the fetal skull bones and fetal skull dimensions is knowledge required for when you review the maternal pelvic bones.
• As the fetal head passes through the maternal pelvis during labour and birth, the fetal skull needs to fit through the maternal pelvis to successfully birth.
• This is called the relationship between the fetal skull bones and the maternal pelvis.