Fetal Skull

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Importance of the fetal skull

  • Protect the brain 
  • Ability to change shape – adapting to the 

process of labour

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Layers of external structure of the skull

  • Skin
  • Connective tissue – containing blood vessels and hair follicles. May become oedematous during labour resulting in caput succedaneum 
  • Aponeurosis – fibrous sheet 
  • Connective tissue – loose layer enabling movement of the scalp 
  • Periosteum – double sheet of connective tissue covering and nourishing the bone. It is attached to the edges of the bone
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The Vault

  • Large dome shaped area, containing the cerebral hemispheres 
  • Bones are relatively thin and pliable to allow them to overlap during labour 
  • It is composed entirely of membrane in early fetal life 
  • At birth the membranes still remain between the bones at the sutures and fontanelles

Points of ossification:

  •  Parietal eminences 
  • Occipital protuberance
  • Frontal bossess
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Bones of the Vault

  • Occipital bone (1) – lies at the back of the head and form the region of the occiput. Part of it forms the base of the skull. At the centre is the Occipital Protuberance. 
  • Parietal Bones (2) – lie on either side of the fetal skull, the centre of ossification of each is the Parietal Eminence. 
  • Frontal Bones (2) – form the forehead (Sinciput). At the centre of each is a Frontal Boss or Eminence. Fuse to become one by the age of 8 years 
  • Temporal Bones (2) – small, flat and form a small part of the vault at the side
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The Base

  • Composed of bones which are firmly united to protect the vital centres in the medulla. 
  • Perforated by a circular opening FORAMEN MAGNUM – through which the spinal cord passes
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The Face

  • Composed of 14 small bones which are firmly united and non compressible
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Fetal skull regions

Face - Extends from the superorbita ridges to the chin

Brow - (sinciput) lies between bregma & root of the  nose 

Mentum - the chin 

Occiput - lies over the occipital bone 

Subocciputal - lies below the occipital protuberance 

Vertex - diamond shaped area between anterior & posterior fontanelles & parietal eminences

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Fetal skull sutures

  • Soft fibrous tissues linking some bones of the skull 
  • Enable moulding of the head during labour 
  • Enables expansion of the brain during childhood 
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Types of fetal skull sutures

Frontal suture - between 2 frontal bones 

Sagittal suture - between 2 parietal bones 

Coronal suture - between parietal & frontal 

Lambdoidal suture - between parietal & occipital 

Temporal suture - between inferior margin of  the parietal & temporal

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Fetal Skull fontanelles

  • Fontanelle – membranous, non ossified area of the skull where three or more sutures meet. 
  •  Anterior fontanelle (Bregma) - diamond shaped space between coronal & sagittal suture, ossifies at 18 months 
  • Posterior fontanelle (lambda) - triangle shaped space between sagittal & lambdoidal suture
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Fetal skull diameters

Biparietal diameter 9.5 cm.

  • between parietal eminences The greatest transverse diameter 

Suboccipitobregmatic 9.5 cm. 

  • measured from the junction of the head with the neck just below the occipital protuberance to the  centre of the anterior fontanelle. 
  •  The presenting diameter of the well flexed head in labour. 

Suboccipitofrontal 10 cm 

  • measured from the junction of the head with the neck just below the occipital protuberance to the centre of  the frontal suture. 
  • The presenting diameter of the partially flexed head.
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Fetal skull moulding

  • Occurs with descent of the fetal head into the pelvis to reduce the head circumference 
  • Frontal bones slip under parietal bones 
  • Parietal bones override each other 
  • Parietal bones slip under the occipital bone
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