Mechanism of normal labor

Mechanism of normal labor

Is the series of passive movement of the fetus in its passage through the birth canal. The skilful management of normal delivery is based on a good knowledge of mechanism of labor.

Terms used in mechanism (movement)

Flexion of the head: Bending of the head over the chest andthe limbs over the abdomen. The head is normally flexed at the beginning of labor, with good uterine contractions flexion of the head is increased, thereby helping descent. The smaller presenting part facilitates descent.

Internal Rotation (of the Head): This is a turning forwardof whatever part of the fetus reaches the pelvic floor first.

Crowning of the head: This is when the occipital eminencepasses under the symphysis pubis and the head no longer recedes between contractions.

Extension: is a movement by which the flexion of the head isundone.

Descent: Downward movement of the presenting part of thefetus. It is aided contraction of the uterus, abdominal muscles, positioning of the fetal body, Amniotic fluid and uterine pressure.

Restitution: This is the turning of the head to undo the twistin the neck which took place during the internal rotation of the head. Usually towards the back of the baby, it reveals the position of the fetus.

Internal rotation of the shoulder: The shoulders engage inoblique diameter of the pelvis. The anterior shoulder reaches the pelvic floor first and rotate forwards, bringing the shoulders into anterior posterior diameter of the pelvic outlet. It takes place during contraction after the head has been born.

External Rotation of the head: This is the turning of thehead which accompanies the internal rotation of the shoulders. That is the occiput turns a further 1/8^th of a circle and it should always be in the same direction as in restitution. The body is ready to be born. Not the same as restitution so should be allowed to occur before the shoulders are born.

Lateral flexion of the body: This is a side ways bending ofthe spines which takes place while the body is being expelled so that it conforms to the curve of the birth canal.

Common Principles to all mechanisms

·              Descent takes place throughout.

·              Which ever part leads and meets the resistance of the pelvic floor relates forwards until it comes under the symphysis pubis.

·              What ever emerges from the pelvic will pivot around the pelvic bone.

Positions of vertex presentations

Mechanism of normal vertex presentation

Left occipito anterior

Right occipito anterior

Left ocicpito anterior – LOA

Lie: is longitudinal

Position: is left occipito Anterior

Presentation; Cephalic

Attitude: One of the complete flexion

Denominator: Occiput

Presenting part: Posterior area of right parietal bone.

Engaging Diameter: Sub-Ocicpito Frontal (10cm).

The occiput faces the left ilio perineal eminence while the sinciput faces the right sacro iliac joint. The sagittal suture lies in the right oblique diameter of the pelvic brim while the shoulders are in the left oblique diameter of the pelvis.

With good uterine contractions descent of the head takes place with increased flexion. The engaging diameter now reduced from sub-occipito frontal (10cm) to sub-occipito bregmatic (9.5cm). The occiput being the leading part reaches the pelvic floor first and rotate 1/8th of a circle forward (along left side of the pelvis. This causes a slight twist in the neck as the head is not in alignment with the shoulders. With further descent the occiput slips beneath the symphysis pubis, crowning occurs, sinciput, face and the chin sweep the perineum and the head is born by extension. Restitution, takes place (the occiput turns towards the left of mother).

The shoulders enter in left oblique diameter of the pelvis, with further descent, the anterior shoulder reaches the pelvic floor first

and rotate 1/8th of a circle forwards along the right side of the pelvis). This internal rotation of the shoulders is accompanied by external rotation of the head. The shoulders are now in anterior posterior diameter of the pelvic outlet. The anterior shoulder slips under the symphysis pubis, the posterior one passes over the perineum and the body is born by lateral flexion towards the mother’s abdomen.

Mechanism for R.O.A.

The some as the L.O.A. but has to substitute right for left and vice versa.