MECHANISM OF LABOUR

 MECHANISM OF LABOUR

IT IS defined as the series of movements that occur on the head in the process of adaptation during in journey through the pelvis

PRINCIPLE OF NORMAL MOVEMENT

1.In the normal labour, the head enters the brim more commonly through the available transverse diameter and to a lesser extent through one of the oblique diameters

2.Left occipto anterior position is commoner than right occipto anterior as the left oblique diameter is encroached by rectum

3.The engaging a tero-posterior diameter of the head is either sub - occipto bregmatic or the sub occipto frontal

4.The principle of movement are

A.ENGAGEMENT 

B.DESCENT 

C.INTERNAL ROTATION 

D. FLEXION 

E. CROWNING 

F.EXTENTION 

G. RESTITUTION

H. EXTERNAL ROTATION 

I. EXPULSION /BIRH OF SHOULDER AND TRUNK

*ENGAGEMENT -

(I) In L. O. A of the vertex, the Fetal head enters pelvic brim with occiput lying in relation to left ileopectineal eminence, sinciput at right sacroiliac joint and sagittal suture lying on right oblique diameter of maternal pelvis

(ii) The engaging transverse diameter of Fetal head is biparietal 9.5 cm and anterior - posterior diameter is sub occipto bregmatic 9.5 cm, both the diameter remain on the same plane

(iii) In ROA, engagement of vertex occur in the left oblique diameter of pelvis with occiput lying at right anterior quadrant

2.DESCENT

IT is a continuous process if there is no bony tissue obstruction. It is slow in fast stage and pronounced in 2nd stage. It is completed with expulsion of Fetus

FLEXION

The resistance of maternal soft passage promote full flexion of Fetal head or the flexion achieved when the head meets the resistance of the birth canal during descent hence the flexion achieved due to the resistance offered by unfolding cervix, wall pelvis, pelvic floor

INTERNAL ROTATION

The mechanism of internal rotation is very complex but easy to describe. In occipto lateral position, there will be anterior rotation by 2/8th circle of occiput whereas in oblique, is 1/8th of circle placing the occiput behind the symphysis pubis

CROWNING

In crowning, the maximum diameter of head (biparietal diameter) stretches the vulvar outlet without any recession of head even after contraction is over called crowning of head

EXTENTION

Delivery of head takes place by extention through couple of force theory. The driving force pushes head in a downward direction while the pelvic floor offers a resistance in upward and forward direction. The downward force and upward force neutralize and remaining forward thus helps in extention

RESTITUTION

IT is visible passive movement of head due to untwisting of neck sustained during internal rotation. Movement of restitution occurs rotating the head through 1/8th of circle in a direction opposite to that of internal rotation

EXTERNAL ROTATION

IT is the movement of the head visible externally due to internal rotation of shoulder

EXPULSION /BIRTH OF SHOULDER AND TRUNK

After shoulder are positioned in anterior - posterior diameter of the outlet, further descent takes place until the anterior shoulder escapes below the symphysis pubis. By the movement of lateral flexion of spine, the posterior shoulder sweeps over the perineum. The rest of trunk is then expelled out by lateral flexion