Fetus

1 anterior fontanelle 2 frontal eminence 3 sinciput 4 orbital ridge 5 glabella 6 femporal bone 7 chin 8 submentobregmatic diameter 9 temporal suture 10 suboccipitobregmatic diameter 11 occipital bone 12 poterolateal fontanelle 13 lambdoidal suture 14 posterior fontanelle 15 parieral eminence 16 coronal suture 17 sagittal suture 18 metopic suture19 biparietal diameter 20 bitemporal diameter

Descent of head

The head of the fetus at the beginning of labor is usually in an occipitotransverse (or occipitolateral) position with a posterior or anterior asynclitism- i.e. the parietal bone presenting. 

As its descent continues to the level of the ischial spines, the occiput rotates at the false pelvis into the left position in most cases. The head is now in a left occipitoanterior position. This initial deflection of the head to the occipitoanterior position is due to the fullness of the sigmoid colon which pushes the head slightly upwards. The pelvic corridor which is a wide opening connecting the abdominal cavity with the false pelvis prevents a similar condition occurring in the other side.

As the head hits the pelvic floor, internal orientation occurs and the head rotates through an angle of 45^o. This rotation brings the head into a direct occipitoposterior position, a position it keeps till it crowns.

Further descent will cause the head to be delivered through a movement of extension so that the occiput is delivered first.

1 fetal head 2 anterior fontanelle 3 posterior fontanelle 4 occiput 5 path of descent of head in labour

As the shoulder enters the pelvic floor and proceeds to pass through the same process as the head, the bisacromial diameter passes through first and becomes the left acromioanterior presentation as a result of the sigmoid (pelvic) colon. It then turns to a direct acromioanterior position at the pelvic floor. At the stage in which the shoulder is at oblique position (i.e. left acromioanterior position) the head is already delivered and it proceeds to undergo restitution by which it rotates through an angle of 45^o to the left in order to be in alignment with the orientation of the shoulder at the pelvis. The head then undergoes external rotation to an occipitolateral postion so that the occiput lies next to the left thigh. At this stage the shoulder has hit the pelvic floor and it has orientated itself into direct anteroposterior position. This is the main reason for the external rotation of the head to the occipitolateral position in order to bring the head in normal alignment with the shoulder.

The anterior shoulder then slips under the symphysis pubis and is delivered. This is followed by the posterior shoulder. The rest of the baby’s body allows the delivery of the shoulders. This normal process of labour may develop complication sat any stage during labour.

 
Pelvic floor