11/13/24, 7\:06 PM Guide | Mechanism of labour

Mechanism of labour

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Introduction

Describing the mechanism of labour is a common topic for OSCEs and MCQs. Although on the surface it can appear
complicated, breaking the process down into individual steps makes it much easier to understand.
Normal labour involves the widest diameter of the fetus successfully negotiating the widest diameter of the bony pelvis of the
mother via the most e
The mechanism of labour covers the passive movement the fetus undergoes in order to negotiate through the maternal bony
pelvis. Labour can be broken down into several key steps.

Key stages of labour

Descent
Engagement
Neck
Internal rotation
Crowning
Extension of the presenting part
Restitution
External rotation
Lateral
For the purposes of this guide, the fetal movements will be described in relation to a cephalic (vertex) presentation with a
longitudinal lie. This is a common (low risk) presentation.
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Pelvic anatomy

To understand the mechanism of labour, you need some basic understanding of pelvic anatomy.

Borders of the pelvic inlet

Posteriorly\: Sacral promontory
Laterally\: Iliopectineal line
Anteriorly\: Pubic symphysis
Pelvic inlet

Borders of the pelvic outlet

Posteriorly\: Tip of the coccyx
Laterally\: Ischial tuberosity
Anteriorly\: Pubic arch

Pelvic dimensions

Transverse diameter Antero-posterior diameter
Pelvic inlet 13cm 11cm
Mid-pelvis 12cm 12cm
Pelvic outlet 11cm 13cm
Since the transverse diameter is greater than the antero-posterior (AP) diameter in the pelvic inlet, the widest
circumference of the fetal head descends in a transverse position. However, when it gets closer to the pelvic outlet, the
nature of the pelvic
position, as the AP diameter is greater than the transverse diameter.
Fetal head diameter varies depending upon the degree of neck
It is also important to know how the circumference of the fetal head varies with di
Suboccipitobregmatic (vertex,
Occipitofrontal (vertex, neutral
Submentobregmatic (face) is 9.5cm
Verticomental (brow) is 13.5cm
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Descent

The fetus descends into the pelvis.
In the primigravida this is likely to occur from 38 weeks gestation onwards, in a multigravida woman, this may not occur until
labour is established.
Descent is encouraged by\:
Increased abdominal muscle tone
Braxton hicks in the late stages of pregnancy
Fundal dominance of the uterine contractions during labour
Increased frequency and strength of contractions during labour
As the head descends, it moves towards the pelvic brim in either the left or right occipito-transverse position (this means the
occiput can be facing the left side or right side of the mother’s pelvis).
Fetal descent

Engagement

This is when the largest diameter of the fetal head descends into the maternal pelvis.
The term engagement is referring to the widest part of the fetal head successfully negotiating its way down deep into the
maternal pelvis. Engagement is identith
palpable or less.
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Fetal engagement

Flexion

As the fetus descends through the pelvis, fundal dominance of uterine contraction exerts pressure down the fetal spine
towards the occiput, forcing the occiput to come into contact with the pelvic
(chin to chest) allowing the circumference of the fetal head to reduce to sub-occipitobregmatic (9.5cm).
In this position, the fetal skull has a smaller diameter which assists passage through the pelvis.
Fetal head

Internal rotation

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The pelvic
right occipito-transverse position a total of 90-degrees, to an occipital-anterior (occiput facing forward) position, to lie under
the subpubic arch.
With each maternal contraction, the fetal head pushes down on the pelvic
e
turn.
This rotation will occur during established labour and it is commonly completed by the start of the second stage. Further
descent leads to the fetus moving into the vaginal canal and eventually, with each contraction, the vertex becomes
increasingly visible at the vulva.
Fetal internal rotation

Crowning

When the widest diameter of the fetal head successfully negotiates through the narrowest part of the maternal bony pelvis,
the fetal head is considered to be ‘crowning’
. This is clinically evident when the head, visible at the vulva, no longer retreats
between contractions. Complete delivery of the head is now imminent and often the woman, who has been pushing, is
encouraged to pant so that the head is born with control.
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Fetal crowning

Extension of the presenting part

The occiput slips beneath the suprapubic arch allowing the head to extend. The fetal head is now born and will be facing the
maternal back with its occiput anterior.
Extension of the fetal head
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External rotation & restitution

Because the shoulders at the point of the head being delivered are only just reaching the pelvic
negotiating the pelvic outlet and the fetus may naturally align its head with the shoulders. This is called restitution and visually
you may see the head externally rotate to face the right or left medial thigh of the mother.
During the next contraction, the shoulders, having reached the pelvic
position to an anterior-posterior position. Evidence of this manoeuvre happening inside can be visualised by seeing the head
externally rotating as the fetus keeps its spine aligned.
Restitution

Delivery of the shoulders & body

Downward traction by the healthcare professional will assist the delivery of the anterior shoulder below the suprapubic arch.
This is followed by upward traction assisting the delivery of the posterior shoulder.
The fetal body will be delivered by the contractions, the health professional's role is only to assist safe negotiation of this last
stage.
Source\: geekymedics.com
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