Mechanism of Labour

Mechanism of Labour (LOA & ROA)

                                                            

Definition

Mechanism of labour is the series of passive movements of the fetus during its passage through the maternal pelvis during labour.

Principles of Mechanism of Labour

1.      Descent takes place through out labour

2.      Whichever part leads and first meets the resistence of the pelvic floor will rotate forward until it comes under the symphysis pubis

3.      Whatever emerges from the pelvis will pivot around the pubic bone.

LEFT OCCIPITO ANTERIOR (LOA)

Lie                               : Longitudinal

Presentation                : Vertex

Position                       : LOA

Attitude                       : Flexion

Denominator               : Occiput

Presenting part            : Posterior part of the right parietal bone.

·         Descent

Descent takes place due to forceful uterine contraction and retraction, rupture of membranes, complete cervical dilatation and maternal efforts.

·         Flexion

Flexion is increased throughout labour. When the head meets the resistance of the pelvic floor flexion is increased. The increased flexion will decrease the presenting diameter ie., sub occipito-frontal (10cm) to a smaller diameter suboccipito-bregmatic (9.5cm). The occiput becomes the leading part.

·         Internal rotation of the head

The occiput leads and meets the pelvic floor first and rotates anteriorly 1/8^th of the circle to come under the symphysis pubis. The anteroposterior diameter of the head now lies in the anteroposterior diameter of the pelvic outlet.

·         Crowning

The occiput slips beneath the subpubic arch and crowning occurs when the head no longer recedes back between the contractions and the widest transverse diameter (biparietal) is born.

·         Extension of the head

The fetal head pivots around the pubic bone while the sinciput, face and chin sweep the perineum and head is born by the movement of extension.

·         Restitution

The twist in the neck of the fetus that resulted from internal rotation is now corrected by a slight untwisting movement. The occiput moves 1/8^th of the circle towards the side from which it started.

·         Internal rotation of the shoulders

The anterior shoulder reaches the pelvic floor first in the left oblique diameter and rotates forward 1/8^th of the circle from right to left and thus the shoulders are now in the AP diameter of the pelvis.

·         External rotation of the shoulders

It takes place simultaneously with internal rotation of the shoulders and the occiput of the fetal head now lies laterally facing mother’s right thigh.

·         Lateral flexion

The anterior shoulder escapes under the symphysis pubis, posterior shoulder sweeps the perineum and body is born by the movement of lateral flexion.

RIGHT OCCIPITO ANTERIOR (ROA)

Lie                               : Longitudinal

Presentation                : Vertex

Position                       : ROA

Attitude                      : Flexion

Denominator               : Occiput

Presenting part            : Posterior part of the left parietal bone.

·         Descent

Descent takes place due to forceful uterine contraction and retraction, rupture of membranes, complete cervical dilatation and maternal efforts.

·         Flexion

Flexion is increased throughout labour. When the head meets the resistance of the pelvic floor flexion is increased. The increased flexion will decrease the presenting diameter ie., sub occipito-frontal (10cm) to a smaller diameter suboccipito-bregmatic (9.5cm). The occiput becomes the leading part.

·         Internal rotation of the head

The occiput leads and meets the pelvic floor first and rotates anteriorly 1/8^th of the circle to come under the symphysis pubis. The anteroposterior diameter of the head now lies in the anteroposterior diameter of the pelvic outlet.

·         Crowning

The occiput slips beneath the subpubic arch and crowning occurs when the head no longer recedes back between the contractions and the widest transverse diameter (biparietal) is born.

·         Extension of the head

The fetal head pivots around the pubic bone while the sinciput, face and chin sweep the perineum and head is born by the movement of extension.

·         Restitution

The twist in the neck of the fetus that resulted from internal rotation is now corrected by a slight untwisting movement. The occiput moves 1/8^th of the circle towards the side from which it started.

·         Internal rotation of the shoulders

The anterior shoulder reaches the pelvic floor first in the right oblique diameter and rotates forward 1/8^th of the circle from left to right and thus the shoulders are now in the AP diameter of the pelvis.

·         External rotation of the shoulders

It takes place simultaneously with internal rotation of the shoulders and the occiput of the fetal head now lies laterally facing mother’s left thigh.

·         Lateral flexion

The anterior shoulder escapes under the symphysis pubis, posterior shoulder sweeps the perineum and body is born by the movement of lateral flexion.