1. MECHANISM OF NORMAL LABOUR
The series of movements that occur on the head in the
process of adaption during its journey through the pelvis, is
called mechanism of labour.
PRINCIPLES COMMON TO ALL MECHANISMS ARE:
Descent takes place throughout the mechanism.
Whichever part leads and first meets the resistance of
the pelvic floor will rotate forwards until it comes under
the symphysis pubis.
Whatever emerges from the pelvis will pivot around the
When these conditions are met, the way that the fetus is normally
situated can be described as follows:
The lie is longitudinal.
The presentation is cephalic.
The position is right or left occiptio-anterior.
The attitude is one of good flexion.
The denominator is the occiput.
The presenting part is the posterior part of the
anterior parietal bone.
2. MAIN MOVEMENTS:
Descent of the fetal head into the pelvis often begins before
the onset of labour. In primigravidae it usually occurs during
the later weeks of pregnancy when engagement of the head
provides confirmation that vaginal delivery is probable. In
multigravidae muscle tone is lax and therefore engagement
3. may not occur until labour actually begins. Throughout the
first stage of labour the forces of contraction and retraction
aid descent. Following rupture of forewaters and full
dilatation of the cervix, maternal effort speeds progress.
This increases throughout labour. The fetal spine is attached
nearer the posterior part of the skull; pressure exerted down
the fetal axis will be more forcibly transmitted to the occiput
than the sinciput. The effect is to increase flexion which
results in smaller presenting diameters which will negotiate
the pelvis more easily. At the onset of labour, the
suboccipito-frontal diameter, 10 cm , is presenting, with
greater flexion the suboccipitobregmatic diameter , 9.5 cm,
presents. The occiput becomes the leading part.
3. INTERNAL ROTATION OF THE HEAD :
During a contraction the leading part is driven downwards
onto the pelvic floor. The slope of the pelvic floor
determines the direction of rotation. The muscles are gutter-
shaped and slope down anteriorly, so whichever part of the
fetus first meets the lateral half of this slope will be directed
forwards and towards the center. In a well flexed vertex
presentation, the occiput leads and meets the pelvic floor
first and rotates anteriorly through one-eight of a circle. This
causes a slight twist in the neck of the fetus as the head is no
longer in direct alignment with the shoulders. The
anteroposterior diameter of the head now lies in the
widest(anteroposterior) diameter of the pelvic outlet,
facilitating an essay escape.
4. 4. CROWNING:
The occiput slips beneath the sub-pubic arch and crowning
occurs when the head no longer recedes between
contractions and the widest transverse
diameter(biparietal)is born. If flexion is maintained, the
suboccipitobregmatic diameter, 9.5 cm, distends the vaginal
5.EXTENSION OF THE HEAD:
Once the crowning has occurred the fetal head can extend,
pivoting on the suboccipital region around the pubic bone. This
releases the sinciput, face and chin which sweep the perineum and
are born by a movement of extension. The suboccipitofrontal
diameter, 10 cm, distends the vaginal outlet.
The twist in the neck of the fetus which resulted from
internal rotation, is now corrected by a slight untwisting movement.
The occiput moves one-eighth of a circle towards the side from
which it started.
7. INTERNAL ROTATION OF THE SHOULDERS:
The shoulders undergo a similar rotation to that of the head
to lie in the widest diameter of the pelvic outlet, namely
anteroposterior. The anterior shoulder is the first to reach the
levator ani muscle and therefore rotates anteriorly to lie under the
symphysis pubis. This movement can be clearly seen as the head
5. turns at the same direction as restitution and the occiput of the fetal
head now lies laterally.
8. LATERAL FLEXION:
The shoulders are born sequentially. The anterior shoulder
slips beneath the sub-pubic arch and the posterior shoulder passes
over the perineum. This enables a smaller diameter to distend the
vaginal orifice than if both shoulders were born simultaneously. The
remainder of the body is born by lateral flexion as the spine bends
sideways through the curved birth canal.
J. Evlin Petescia
Mahima Susan Ajith