Obstetrics Simplified - Diaa M. EI-Mowafi
Anatomy of the Foetal Skull
It consists of vault, face and base.
The vault is composed of:
- 2 frontal bones separated by the frontal suture,
- 2 parietal bones separated by the sagittal suture,
- the occipital bone separated by the lambdoidal suture from the parietal bones, while the coronal suture separates the frontal from the parietal bones.
Each of the 2 parietal bones is separated from the temporal bone on each side by the temporal suture.
The face is the area from the junction of the chin and neck to the root of the nose and supra-orbital ridges.
The vertex is the area of the vault bounded ;
- anteriorly by the anterior fontanelle and the coronal suture,
- posteriorly by the posterior fontanelle and lambdoidal suture,
- laterally by 2 lines passing by the parietal eminencies.
The brow is the area from the nose and supra-orbital ridges to the anterior fontanelle and coronal suture.
The Fontanelles
These are 6 areas lie at the meeting of the sutures. Four fontanelles lie at the anterior and posterior end of the temporal sutures on each side and have no obstetric importance. The anterior and posterior fontanelles are important to diagnose:
- the vertex presentation,
- the position of the occiput,
- the degree of flexion of the head.
| Anterior Fontanelle (Bregma) | Posterior Fontanelle (Lambda) |
| Large, and lozenge-shaped. | Small and triangular. |
| Its floor is membranous. | Its floor is bony. |
| Surrounded by 4 bones. | Surrounded by 3 bones. |
| (2 frontal and 2 parietal). | (2 parietal and occipital). |
| The floor is completely ossified 1.5 years after birth. | The floor is completely ossified at full term. |
| The surrounding bones are not overlapping during moulding. | The surrounding bones are overlapping during moulding. |
Diameters of Foetal Skull
Longitudinal diameters
- Suboccipito-bregmatic = 9.5cm
- from below the occipital protuberance to the centre of the anterior fontanelle (bregma).
- It is the engagement diameter in occipito-anterior with complete flexion.
- Suboccipito-frontal = 10 cm
- from below the occipital protuberance to the anterior end of the bregma.
- It is the engagement diameter in occipito anterior with incomplete flexion.
- It is the diameter that distends the vulva in occipito anterior if the head is allowed to extend after crowning.
- Occipito-frontal = 11.5 cm
- form the occipital protuberance to the root of the nose.
- It is the engagement diameter in occipito-posterior position.
- It is the diameter that distends the vulva in face to pubis delivery.
- It is the diameter that distends the vulva if the head extends before crowing in occipito anterior.
- Submento-bregmatic = 9.5 cm
- from the junction of the chin and neck to the centre of the bregma.
- It is the engagement diameter in face presentation when the head is completely extended.
- Submento-vertical =11.5 cm
- from the junction of the chin and neck to the vertical point which is a point on the sagittal suture midway between anterior and posterior fontanelles.
- It is the engagement diameter in the incompletely extended face.
- It is the diameter that distends the vulva during face delivery.
- Mento-vertical =13.5 cm
- from the tip of the chin to the vertical point.
- It is the engagement diameter in brow presentation. As it is longer than the largest diameter of the pelvic brim, the head cannot enter the pelvis.
Transverse diameters
- Biparietal =9.5 cm
- between the 2 parietal eminencies.
- Subparietal supraparietal = 9cm
- from below one parietal eminence to above the opposite eminence.
- It is the engagement diameter in case of asynclitism.
- Bitemporal = 8 cm
- between the anterior ends of the temporal sutures./li>
- Bimastoid = 7.5cm
- between the tips of the 2 mastoid processes.