The mechanism of labor involves a series of changes in the position and attitude of the fetus as it passes through the birth canal. As labor progresses, the fetal head engages in the pelvis, descends further, and flexes to minimize its diameter. The head then undergoes internal rotation so the occiput rotates under the pubic bone. Further descent and extension occurs until the head crowns. Restitution and external rotation align the shoulders for delivery followed by delivery of the shoulders and body.

1. Mechanism of Labor
By:
Khair-Ullah Khan Barak
4th year MBBS
Hamdard University, Karachi, Pakistan

2. Labor
Definition:
Process by which regular painful
contractions bring about effacement and
dilation of the cervix and descent of the
presenting part, ultimately leading to expulsion
of the fetus and the placenta from the mother.
Consists of three stages

3. Mechanism of Labor
Refers to the series of changes in position and
attitude that the fetus undergoes during its
passage through the birth canal.
The relation of the fetal head and body to the
maternal pelvis changes as fetus descends
through the pelvis.
This is essential so that the optimal diameters
of the fetal skull are present at each stage of
the descent.

4. Series of changes during Labor
Engagement
Descent
Flexion
Internal rotation
Extension
Restitution
External rotation
Delivery of the shoulders and fetal body

5. Engagement
Engagement is said to have occurred
when the widest part of the
presenting part has passed
successfully through the pelvic inlet.
Head is normally in the transverse
position.
Prior to labor in nulliparous women.
During labor in multiparous women.
If more than two-fifths of the fetal
head is palpable abdominally, the
head is not yet engaged.

6. Descent
Is needed before Flexion, Internal rotaion
and Extension
Secondary to uterine action during 1st
stage and latent phase of 2nd stage of
labor.
Helped by voluntary use of abdominal
muscles and valsalva maneuver during
active phase of 2nd stage of labor

7. Flexion
Occurs as the head
descends into the
narrower mid-cavity
of the pelvis.
This occurs passively
due to the
surrounding
structure.
Important in
minimizing the
presenting diameter
of the fetal head.

8. Internal rotation
Normal= Occipito – anterior
After completion the occiput is
underneath the symphysis pubis and
bregma is near lower border of the
sacrum.
Some times occipito – posterior but
changes to occipito – anterior .
If occipito – posterior persists then there
would be a need for instrumental
delivery or even Caesarean section.Face to pubes

9. Extension
 The well flexed head now extends and the occiput escapes from underneath the symphysis
pubis and distends the vulva.
 This is known as crowning of the head.
 Further extension occurs and the occiput underneath the symphysis pubis acts as fulcrum
point as the bregma, face and chin appear in succession over the post vaginal opening and
perineal body resulting in minimizing soft tissue trauma.

10. Restitution
The slight rotation of the occiput through one – eight of the circle.
When the head is delivering the occiput is directly anterior.
As soon as it escapes from the vulva, the head aligns itself with
shoulders, which have entered the pelvis in oblique position.

11. External rotation
In order to be delivered, the shoulders have to rotate into the direct
AP plane as the pelvis is widest at its outlet.
When this occurs, the occiput rotates through a further one – eight
of a circle to the transverse position.

12. Delivery of the shoulders and fetal body
 The anterior shoulder will be under
symphysis pubis and delivers 1st.
 The posterior shoulder delivers 2nd.
 This process may occur without assistance, but lateral traction is often exerted by gently pulling
the fetal head in a downward direction to help release the anterior shoulder from underneath
the pubic symphysis

13. Normally rest of the
body is delivered easily,
with the posterior
shoulder guided over
the perineum by
traction in the opposite
direction, so sweeping
the baby on to the
maternal abdomen