The ability of the fetus to successfully negotiate the pelvis during labor involves changes in the position of its head during its passage in labor. The mechanisms of labor, also known as the cardinal movements, are described in relation to a vertex presentation, as is the case in 95% of all pregnancies.

1. Page 1 of 11
MECHANISM OF LABOUR Sharma
TO GET FURTHER MORE NOTES:
https://sharmanotes.blogspot.com/
Prasad Sharma VG
MECHANISM
OF
LABOUR

2. Page 2 of 11
MECHANISM OF LABOUR Sharma
A 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.
The series of movements that occur on the head in the process of adaptation during its journey
through the pelvis, is called Mechanism of labour.
 It should be borne in mind that while the principal in or initiating the movement.
 Movements are taking place in the head, the rest of the fetal trunk is also involved in it,
either participating in or initiating the movement.
In normal labour, the head enters the brim more commonly through the available transverse
diameter (70 %) and to a lesser extent through one of the oblique diameters.
 Accordingly, the position is either occipito-lateral or oblique occipito-anterior.
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
MECHANISM OF NORMAL LABOUR
DEFINITION:
MECHANISM OF NORMAL LABOUR:

3. Page 3 of 11
MECHANISM OF LABOUR Sharma
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 13 cm 11 cm
Mid-Pelvis 12 cm 12 cm
Pelvic Outlet 11 cm 13 cm
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

4. Page 4 of 11
MECHANISM OF LABOUR Sharma
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.
DESCENT:
Descent is encouraged by:
 Increased abdominal muscle tone
 Braxton hicks in late stages of pregnancy
 Increased frequency and strength of contractions during labour
 Straightening of the fetal ovoid specially after rupture of the membranes.
Mechanism:
 Provided there is no undue bony or soft tissue obstruction, descent is a continuous
process.
 It is slow or insignificant in first stage but pronounced in second stage.
 It is completed with the expulsion of the fetus.
 In primigravidae, with prior engagement of the head, there is practically no descent in
first stage, while in multipurae, descent starts with engagement.
Descent Engagement Neck flexion Internal rotation
CrowningRestitution
Internal rotation
 g
Lateral flexion
 g
Extension
STAGES OF LABOUR

5. Page 5 of 11
MECHANISM OF LABOUR Sharma
 Head is expected to reach the pelvic floor by the time the cervix is fully dilated.
ENGAGEMENT
 In others, the sagittal suture lies more posteriorly with the result that the anterior parietal
bone becomes the leading presenting part and is then called anterior parietal presentation
or anterior asynclitism. Fig A
 Deflected anteriorly towards the symphysis pubis or posteriorly towards the sacral
promontory, such deflection of the head in relation to the pelvis is called asynclitism. Fig B
 When the sagittal suture lies interiorly, the posterior parietal bone becomes the leading
presenting part and is called posterior asynclitism or posterior parietal presentation. Fig C
 This is more frequently found in primigravidae because of good uterine tone and a tight
abdominal wall.

6. Page 6 of 11
MECHANISM OF LABOUR Sharma
 After this movement which occurs early in labour, not only the head enters the brim but
synclitism occurs.
 However, in about 25 % of cases, the head enters the brim in synchitism i.e. the sagittal
suture corresponds to the diameter of engagement.
 In primigravidae, engagement occurs in a significant number of cases before the onset of
labour.
 While in multiparae, the same may occur in late first stage with rupture of the membranes.
NECK 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 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).
 While some degree of flexion of the head is noticeable at the beginning of labour but
complete flexion is rather uncommon.
 In this position, the fetal skull has a smaller diameter which assists passage through the
pelvis.
 If the pelvis is adequate, flexion is achieved either due to the resistance offered by the
unfolding cervix, the walls of the pelvis or by the pelvic floor.
 Flexion is essential for descent, since it reduces the shape and size of the plane of the
advancing diameter of the head.

7. Page 7 of 11
MECHANISM OF LABOUR Sharma
INTERNAL ROTATION
 It is a movement of great importance without which there will be no further descent.
 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.
 After the contraction passes off, elastic recoil of the levator ani occurs bringing the
occiput forwards towards the midline. The process is repeated until the occiput is placed
anteriorly. This is called rotation by law of pelvic floor (Hart’s rule).
 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.
 In occipito-lateral position there will be anterior rotation by 2/8th of a circle of the
occiput.
 In obligue anterior position, rotation will be 1/8th of a circle forward, placing the occiput
behind the symphysis pubis.
 There is always an accompanying movement of descent with internal rotation
 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.

8. Page 8 of 11
MECHANISM OF LABOUR Sharma
CROWNING:
 After internal rotation of the head, further descent occurs until the subocciput lies
underneath the pubic arch.
 At this stage, the maximum diameter of the head ( biparietal diameter ) stretches the
vulval outlet without any recession of the head even after the contraction is over — called
" crowning of the head " .
EXTENSION:
 Delivery of the head takes place by extension through “couple of force” theory.
 The driving force pushes the head in a downward direction while the pelvic floor offers a
resistance in the upward and forward direction.
 The downward and upward forces neutralise and remaining forward thrust helping in
extension.
 The successive parts of the fetal head to be born through the stretched vulval outlet are
vertex, brow and face.
 Immediately following the release of the chin through the anterior margin of the
stretched perineum, the head drops down, bringing the chin in close proximity to the
maternal anal opening.

9. Page 9 of 11
MECHANISM OF LABOUR Sharma
RESTITUTION:
 It is the visible passive movement of the head due to untwisting of the neck sustained
during internal rotation.
 Movement of restitution occurs rotating the head through 1/8th of a circle in the
direction opposite to that of internal rotation
 The occiput thus points to the maternal thigh of the corresponding side to which it
originally lay.
EXTERNAL ROTATION:
 It is the movement of rotation of the head visible externally due to internal rotation of the
shoulders.
 As the anterior shoulder rotates towards the symphysis pubis from the oblique diameter,
it carries the head in a movement of external rotation through 1 / 8th of a circle in the
same direction as restitution.
 The shoulders now lie in the antero - posterior diameter.
 The occiput points directly towards the maternal thigh corresponding to the side to which
it originally directed at the time of engagement.

10. Page 10 of 11
MECHANISM OF LABOUR Sharma
BIRTH OF SHOULDERS AND TRUNK:
 After the shoulders are positioned in antero-posterior diameter of the outlet, further
descent takes place until the anterior shoulder escapes below the symphysis pubis first.
 By a movement of lateral flexion of the spine, the posterior shoulder sweeps over the
perineum. Rest of the trunk is then expelled out by lateral flexion.

11. Page 11 of 11
MECHANISM OF LABOUR Sharma
REFERENCES:
 DC Dutta's Textbook of Obstetrics, Author: D. C. Dutta, Editor: Hiralal Konar,
Publisher: Jaypee Brothers Medical Publishers. Chapter 12: Normal Labour, Page No:
125-129.
 https://geekymedics.com/mechanism-of-labour/