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Mechanisms of Labour and birth Cheat Sheet by

Mechanisms of Labour and birth

Mechanism basics

The baby engages in the widest diameter of the pelvic inlet = Transverse (13cm)
Enters the circular cavity (12cm) and rotates
Exits via the widest diameter the antero­-po­sterior (AP) of the pelvic outlet (13cm)

Mechanisms of Labour

The birth canal is formed by the bony pelvis and soft tissues
To negotiate the birth canal, the fetus must take advantage of the widest diameter of each part of the pelvis
The baby needs to twist and turn as it makes its way through the birth canal and adapt to the shape of the mothers pelvis
The pelvic inlet is widest in the transverse diameter and the pelvic outlet is widest in the antero­pos­terior

The 'normal' mechanism requires that the fetal:

Lie is longit­udinal
Attitude is fully flexed
Presen­tation is cephalic
Position is right or left occipi­toa­nterior
Presenting part or Denomi­nator is the occiput

According to Rankin

The fetus descends
The presenting part of the fetus meets resistance of the pelvic floor
The presenting part rotates forwards to lie anteriorly under the symphysis pubis
The emerging part of the fetus pivots around the pubic bone

Causes of descent

Descent occurs due to: uterine contra­­ct­ions, amniotic fluid pressure, abdominal muscle contra­­ctions
Descent is encouraged by: Increased abdominal muscle tone, Braxton hicks, fundal based uterine contra­­­c­t­ions, increased frequency and strength of contra­­­c­tions

Mechanisms of labour movements

- Descent
- Flexion
- Internal rotation of the head
- Crowning and extension of the head
- Restit­ution
- Internal rotation of the shoulders and external rotation of the head
- Lateral flexion

Descent - the fetus descends into the pelvis

As the head descends, it moves towards the pelvic brim in either the left or right occipi­tot­ran­sverse position
The baby descends through the pelvic inlet towards the pelvic floor
As the fetus descends through the pelvis, it changes position to accomm­odate the widest part of the woman's pelvis


Contra­ctions push down on the spine causing the fetal head to come in contact with the pelvic floor which leads to flexion of the fetal head.
The increased flexion leads to the presen­tation of the smallest diameter of the fetal head (sub-o­ccipito bregma­tic), which assists passage through the pelvis.

Internal rotation of the head

Internal rotation of the head aligns the head with the antero­pos­terior diameter of the pelvic outlet and moves the occiput forwards to lie under the symphysis pubis
This is caused by pressure from uterine contra­ctions, the shape of the pelvic floor, the pressure of the ischial spines
It causes a slight twisting of the neck and now the head is no longer in line with the shoulders

Crowning and extension of the head

The occiput now moves from beneath the pubic arch and pushes against the vaginal entrance
The head is born by extension and pivots on the subocc­ipital region around the pubic bone
The sinciput, face and chin sweep the perineum


Restit­ution occurs after the birth of the head
The head returns to its original position in correct alignment with the shoulders

Internal rotation of the shoulders

This occurs when the shoulders turn to fit the widest diameter of the pelvic outlet (anter­opo­ste­rior)
The anterior shoulder reaches the pelvic floor and rotates forwards to lie under the symphysis pubis
This causes external rotation of the head so the occiput lies laterally towards the woman's thigh
The anterior shoulder is usually born first under the pubic arch

Lateral flexion

The spine undergoes lateral flexion to accomm­odate the curved birth canal and the rest of the body is born in that position


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