Fetal Skull

AREAS OF SKULL:

• Vertex : Quadrangular area bounded anteriorly by the bregma and coronal sutures behind by the lambda and lambdoid sutures and laterally by lines passing through the parietaleminences.
• Brow : Bounded on one side by the anterior fontanel and coronal sutures and on  other side by root of the nose and supraorbital ridges of either side.
• Face :Bounded on one side by root of the nose and supraorbital ridges & other, by junction of foor of mouth with neck.
• Sinciput:Lie in  front of the anterior fontanel

SUTURES:

• Sagittal or longitudinal suture: Between two parietal bones.
• Coronal sutures:Between parietal and frontal bones on either side(Close:30-35yrs)
• Frontal suture :Between two frontal bones.
• Lambdoid sutures :Separate occipital bone and the two parietal bones.
• Metopic suture:calvarial suture on  frontal bone from the nasion to the bregma across midline (Closes:6yrs)

SIGNIFICANCE:

• Permits gliding movement of one bone over other during delivery
• Digital palpation of sagittal suture:Manner ofengagement of the head (asynclitism or synclitism), degree of internal rotation of the head and degree of molding

FONTANELS:

• Wide gap in the suture line is called fontanel(6)

2 of  obstetric significance:

Anterior fontanel or bregma:

• Formed by anteriorly frontal, posteriorly sagittal and on either side, coronal
• Diameter:3 cm 
• Ossification:18 months after birth.
• Denotes the degree of !exion of the head
• Facilitates molding of the head.
• Accommodate marked brain growth
• Refects intracranial status:Depressed in dehydration, elevated in raised ICT
• Collection of blood and exchange transfusion
• CSF can be drawn
• Anterior fontanelle and supraobital ridge is felt in the second stage of labour in Brow presentation

Posterior fontanel:

• Junction of sagittal suture anteriorly & lambdoid suture on either side
• Measures about 1.2 × 1.2 cm
• Its floor is membranous but becomes bony at term
• It denotes the position of the head in relation to maternal pelvis

Sagittal fontanel:

• Situated on the sagittal suture at the junction of anterior two-third and posterior one-third

DIAMETERS OF SKULL:

• The anteroposterior diameters of the head which may engage are:

Diameters	Measurement in Cm (inches)	Attitude of the Head	Presentation
Suboccipitobregmatic	9.5 cm (3 ¾”)	Complete fexion	Vertex
Suboccipito-frontal	10 cm (4″)	Incomplete fexion	Vertex
Occipitofrontal	11.5 cm (4 ½”)	Marked deflexion	Vertex
Mento-vertical	14 cm (5 ½”)	Partial extension	Brow
Submentovertical	11.5 cm (4 ½”)	Incomplete extension	Face
Submentobregmatic	9.5 cm (3 ¾”)	Complete extension	Face

• The transverse diameters:

Diameters	Measurement in Cm (inches)
Biparietal	9.5 cm(3 ¾”)
Super-subparietal	8.5 cm (3 ½”)
Bitemporal	8 cm (3 ¼”)
Bimastoid	7.5 cm (3″)

CIRCUMFERENCES:

Circumferences of the Head in Different Attitudes:

Attitude of the Head	Plane of Engagement	Circumference
Complete fexion	Biparietal-suboccipitobregmatic (almost round shape)	27.5 cm (11″)
Deflexed	Biparietal-occipito-frontal (oval shape)	34 cm (13 ½”)
Incomplete extension	Biparietal-mento-vertical (bigger oval shape)	37.5 cm (15″)
Complete extension	Biparietal-submentobregmatic (almost round shape)	27.5 cm (11″)

MOLDING:

• Alteration of the shape of fore-coming head while passing through the resistant birth passage during labor.
• During normal delivery, an alteration of 4 mm in skull diameter commonly occurs

Mechanism:

• Compression of the engaging diameter of the head with corresponding elongation of diameter at right angle to it.
• In well flexed head of the anterior vertex:engaging suboccipitobregmatic diameter is compressed with elongation of head in mento-vertical diameter
• Parietal bones overlap  occipital bone, frontal bones & temporal bones
• In first vertex position:Right parietal bone tends to override the left & reverse in second vertex position
• Molding disappears within few hours after birth.

Grading:

• Grade-1:Bones touching but not overlapping
• Grade-2: Overlapping but easily separated 
• Grade-3:Fixed overlapping

Importance:

• Slight molding is inevitable and beneficial. 
• Enables head to pass more easily, through birth canal.
• Extreme molding as met in disproportion may produce severe intracranial disturbance in form of tearing of tentorium cerebelli or subdural hemorrhage.
• Shape of the molding can be a useful information about the position of the head occupied in the pelvis.

• Formation of Diffuse, boggy swelling due to stagnation of sero-sanguineous fluid in the layers of the scalp beneath girdle of contact crossing midline suture.
• Disappears spontaneously within 24 hrs after birth.

Mechanism of formation:

• Head descends→ press over dilating cervix or vulval ring →tissues in contact with full circumference of the girdle of contact is compressed→venous return and lymphatic drainage interrupted→stagnation of fluid
• Signifies static position of head
• Gives an idea about the position of the head occupied in the pelvis & degree of flexion achieved

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