FETAL SKULL

Fetal skull

Introduction:  The fetal head, from an obstetrical viewpoint, and in particular its size, is important because an essential feature of labor is the adaptation between the fetal head and the maternal bony pelvis.

LANDMARK OF FETAL SKULL 

·         Occiput:- is the occipital bone/external occipital protuberance.

·         Sinciput:- is the forehead region of fetal head.

·         Parietal eminences:- are the eminences of parietal bone on either side.

·         Frontal eminence: are the eminences of frontal bone on either side.

·         Occipital eminence: are the eminences of occipital  bone

·         Mentum:- is the chin.

·         Vertical point: - is the centre of sagittal suture.

·         Frontal point: - is the root of nose.

·         Sub occiput: - it is the junction fetal neck and occiput.

·         Sub mentum:- it is the junction between neck and chin.

·         Bi parietal:- is the transverse distance between two parietal eminences.

·         Bi temporal: - is the distance between two lower end of coronal suture

AREA OF THE FETAL SKULL:

Fetal skull made up of vault, face and base.

·         Vault:  It is the quadrangular area bounded anteriorly by the bregma and coronal sutures behind by the lambda and the lambdoid sutures and laterally by the line passing through the parietal eminences. It is the commonest presentation

·         Base/Brow:-  It is an area bounded on one side by the anterior fontanelle and the coronal sutures and on the other side by the root of the nose and supra-orbital ridges of the either side.

·         Face: - It is an area bounded on one side by the root of the nose and the supra-orbital ridges and on the other by the junction of the floor of mouth with neck.

DIVISION OF VAULT:

Forehead: from Bregma to root of the nose.

Vertex: bounded with anteriorly anterior fontanels. Posteriorly by posterior fontanels and laterally by parital eminence.

Occiput: from posterior fontanelle to external occipital protuberance.

BONES OF THE VAULT:

These bones makes valt of the fetal skull.

·         Frontal bomes-2

·         Perital bones-2

·         Occipal bone-1

·         Temporal bones-2

SUTURES OF THE FETAL SKULL:

Membranous space between the skull bones of foetus is known as sutures.

·         Frontal suture(Metopic): between two frontal bones

·         Sagital suture(longitudinal): between two parital bones

·         Coronal suture: between two parital and two frontal bones

·         Lambdoid suture: between two parital and occipal bones.

·         Two temporal suture:

FONTANEL’S OF THE FETAL SKULL:

Wide gap in the suture line where 2 or more than 2 sutures meet is called fontanels. 

They are of great obstetric importance.

They are

·         Anterior fontanels (Bregma):  a diamond-shaped membrane-filled space located between the two frontal and two parietal bones of the developing fetal skull. Closes approximately at 18-24 months. Diameter is 3-4cm length and 2-3 cm width.

·         Posterior fontanel’s (Lambda) : is triangle-shaped. It lies at the junction between the sagittal suture and suture. Closes approximately at 1 ½ -2 months. Diameter is 1.2 cm length and 1.2 cm width.

Other fontanels which have less importance in obstetrics are:

·         Mastoid fontanels (posterolateral fontanels): between the temporal, occipital, and parietal bones. Closes next from 6 to 18 months after birth

·         Sphenoid fontanel’s (Anteriolateral fontanels):  between the sphenoid, parietal, temporal, and frontal bones. Next to close around 6 months after birth;

DIAMETERS OF THE FETAL SKULL:

1.      Anterio=posterior diameter:

·         Suboccipitobregmatic=9-9.5cm  (from suboccipital point to the center of  center of anterior fontanelle)

·         Suboccipito frontal= 10cm   ( from suboccipital point to anterior end of the Bregma)

·         Occipito frontal=11cm (occipital protuberance to the root of the nose)  Mentovertical=13-14cm(largest diameter) (from the tip of the  chin to the vertical point)

·         Submentovertical= 11.25-11.5cm (from the junction of the neck and chin to the centre of the vertical point)

·         Submentobregmatic=9.5cm (from the junction of the neck and chin to the centre of thebregma)

2.      Transvers diameter of the fetal skull:

·         Biperital =9.5 (common type) (between the 2 parital eminence)

·         Super sub perital= 8.5cm (from above the parital eminace to the below the opposite eminence)

·         Bitemporal= 8cm (between the anterior end of the temporal suture)

·         Bimastoid= 7.5cm (smallest diameter)  (between the tip of the 2 mastoid processes)

3.      Other diameters for delivery of body of the foetus is:

·         Bitrochanteric =12cm (between the two acromian processes)

·         Bisacromial=10cm (between the 2 iliac crust)

IMPORTANT DEFINITIONS:

Moulding: It is the alteration in the shape of the fore coming head while passing through the resistance of birth passage during labours.

Mechanism:-   Overlapping of cranial bones at the membranous joints due to compression of the engaging diameter of the head. It is physiological, harmless and disappears within a few hours after birth.

Attitude: used to describe the degree of flexion or extension of the head on the neck
 Presentation Refers to which anatomical part of the foetus is leading, that is, closest to the pelvic inlet of the birth canal.

Lie: the relation between the longitudinal axis of the foetus to that of the mother.

Position: the relation of the back of the foetus to the right or to the left side of the mother and whether it is directed anteriorly or posteriorly.