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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 efficient route.
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
Neck flexion
Internal rotation
Extension of the presenting part
Internal rotation
Lateral flexion
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.
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 outlet
Pelvic dimensions
Transverse diameter Antero-posterior diameter
Pelvic inlet 13cm
Mid-pelvis 12cm
Pelvic outlet 11cm
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 floor muscles
encourages the fetal head to rotate from a transverse position to an anterior-posterior
position, as the AP diameter is greater than the transverse diameter.
Fetal head diameter varies depending upon the degree of neck flexion
It is also important to know how the circumference of the fetal head varies with different degrees
of neck flexion:
Suboccipitobregmatic (vertex, flexed) is 9.5cm
Occipitofrontal (vertex, neutral flexion) is 11.0cm
Submentobregmatic (face) is 9.5cm
Verticomental (brow) is 13.5cm
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 occipitotransverse position (this means the occiput can be facing the left side or right side of the
mother’s pelvis).
Fetal descent
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 identified by abdominal palpation,
where the fetal head is 3/5th palpable or less.
Fetal engagement
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 floor. When this occurs the fetal neck flexes (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 flexion
Internal rotation
The pelvic floor has a gutter shape with a forward and downward slope, encouraging the fetal
head to rotate from the left or 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 floor. Following
each contraction, a rebound effect supports a small degree of rotation. Regular contractions
eventually lead to the fetal head completing the 90-degree 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
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.
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
External rotation & restitution
Because the shoulders at the point of the head being delivered are only just reaching the pelvic
floor they are often still 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 floor, will complete their
rotation from a transverse 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.
External rotation of shoulders to an antero-posterior position
External rotation of shoulders to an antero-posterior position
Delivery of the shoulders and 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.
Downward traction
Delivery of the anterior shoulder
Upward traction
Delivery of the posterior shoulder