Mechanism of left sacro anterior:
Attitude: complete flexion/ compaction
Position: left/right sacro anterior
Presenting part: anterior (left/right) buttock
The diameter of engagement of the buttocks is one of the oblique diameter of the inlet. The engaging diameter is bi-trochanteric (10cm) with sacrum directed towards the ilio-pubic eminence. When the diameter passes through the pelvic brim, breech is engaged
Compaction: Descent takes place with increased compaction, due to increased flexion of the limbs.
Internal rotation of buttocks: The anterior buttock reaches the pelvic floor first and rotates forwards 1/8th of a circle along the right side of the pelvis to lie underneath the symphysis pubis. The bi-trochanteric diameter is now in the anteroposterior diameter of the outlet.
Lateral flexion of the body: the anterior buttock escapes under the symphysis pubis, the posterior buttocks sweeps the perineum and the buttocks are born by movement of lateral flexion.
Delivery of the trunk and the lower limbs follow.
Restitution of the buttocks: The anterior buttock turns slightly to the mother's right side.
Internal rotation of the Shoulders : the shoulders enters the pelvis in the same oblique diameters as the buttocks, the left oblique. The exterior shoulder rotates forwards 1/8 th of the circle along the right side of the pelvis and escapes under the symphysis pubis, the posterior shoulder sweeps the perineum and the shoulders are born.
Internal rotation of the head: the head enters the pelvis with the sagital suture in the transverse diameter of the brim. The occiput rotates forwards along the left side and the sub occipital region (the nape of the neck) impinges on the under surface of the symphysis pubis.
External rotation of the body: at the same time the body turns so that the back is uppermost.
Birth of the head: The chin, face, sinciput sweeps the perineum and the head is born in a flexed attitude.