Diameters of Skull

Diameters of Skull-
The engaging diameter of the fetal skull depends on the degree of flexion present.-

 

Diameters Measurement Attitude of the Head Presentation

Suboccipitobregmatic -

Extends from the nape of the neck to the center of the bregma

9.5 cm  Complete flexion
 
Vertex

Suboccipito-frontal -

Extends from the nape of the neck to the anterior end of the anterior fontanel or center of the sinciput

10 cm Incomplete flexion
 
Vertex

Occupitofrontal - 

Extends from the occipital eminence to the root of the nose (Glabella)

11.5 cm Marked deflexion
 
Vertex

Mento-vertical -

 Extends from the midpoint of the chin to the highest point on the sagittal suture

14 cm Partial extension
 
Brow

Submentovertical - 

Extends from junction of floor of the mouth and neck to the highest point on the sagittal suture

11.5 cm Incomplete extension Face

Submentobregmatic -

Extends from junction of floor of the mouth and neck to the center of the bregma

9.5 cm Complete extension
 
Face

The transverse diameters which are concerned in the mechanism of labor are

Biparietal diameter (9.5 cm )

It extends between two parietal eminences. Whatever may be the position of the head, this diameter nearly always engages.

Super-subparietal (8.5 cm)

 It extends from a point placed below one parietal eminence to a point placed above the other parietal eminence of the opposite side.

 Bitemporal diameter (8 cm)

 It is the distance between the anteroinferior ends of the coronal suture.

 Bimastoid diameter ( 7.5 cm)

It is the distance between the tips of the mastoid processes. The diameter is incompressible and it is impossible to reduce the length of the bimastoid diameter by obstetrical operation. 

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