Importance of the fetal skull
- Protect the brain
- Ability to change shape – adapting to the
process of labour
Layers of external structure of the skull
- 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
- 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
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
- 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
- Composed of 14 small bones which are firmly united and non compressible
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
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
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
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
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.
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