Postition of fetus and Mechanism of labor

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POSITION
OF FETUS
AND
MECHANISM
OF LABOR
By NK

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Goals
 Position of fetus in uterus :- Lie, presentation,
presenting part, attitude, denominator, position.
 Mechanism of labor :- definition
 Steps of labor of occipito lateral position :-
engagement, descent, flexion, internal rotation,
crowning, extension, restitution, internal rotation
of shoulder & external rotation of head, expulsion
of shoulder & trunk

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LIE
 Relationship of the
long axis of the
fetus to the long
axis of centralized
uterus or maternal
spine.
 Longitudinal –
99.5%
 Transverse, oblique
or unstable – 0.5%

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PRESENTATION
 Part of the fetus which occupies lower pole of the uterus (pekvic brim).
 Cephalic (head) -96.5%
 Breech / podalic (feet) – 3%
 Shoulder – 0.5%
PRESENTING PART
 Part of the presentation which overlies the internal os & felt by
examining finger through cervical opening.
 Cephalic presentation – vertex 96%, brow & face 0.5%
 Breech – fetal legs. Complete breech – flexed. Frank breech –
extended. Footling – foot.

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ATTITUDE
 Relation of the different parts of fetus
to one another.
 Universal attitude – flexion.
 During later months – head, trunk &
limbs of fetus maintain attitude of
flexion on all joints – form ovoid
mass, corresponds to ovoid uterus.
 Extension may occur – deflexed
vertex, brow or face
 Legs may extend in breech
Diagrammatic representation of a
fetus in flexed attitude
P–V—Vertico-podalic diameter;
A–A—Bisacromial diameter;
T–T—Bitrochanteric diameter

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Varieties of cephalic presentations in
different attitude

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DENOMINATOR
 Arbitrary bony fixed point on presenting part, comes in relation
with various quadrants of maternal pelvis.
 Occiput in vertex, mentum (chin) in face, frontal eminence in
brow, sacrum in breech & acromion in shoulder.
POSITION
 Relation of denominator to different quadrants of pelvis. Pelvis is
divided into equal segments of 450 – 8 positions.
 Anterior or posterior or oblique - left or right.
 First vertex – LOA, second – ROA, third – ROP. Fourth – LOP.

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The position and relative frequency of the vertex at
the onset of labor

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Causes of preponderance of longitudinal
lie and cephalic presentation
 Fetus in attitude of flexion, a shape of an ovoid – long vertico-podalic axis, 25cm at
term.
 Fetus accommodates comfortably along the long axis of ovoid shape of the uterine
cavity at term – longitudinal lie.
 Cephalic presentation – majority. Due to :-
 Gravitation – head being heavier comes down
 Adaptation – smallest circumference of flexed head, 27.5cm & circumference of
complete breech (flexed), 32.5cm. Cephalic & podalic poles comfortably
accommodated in narrow lower pole & wider fundal pole of uterus respectively.

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DEFINITION
 Series of movements that occur on the head in the process of adaptation, during
journey through the pelvis.
 Principle movements take place in the head and rest of fetal trunk involved in
participation or initiation of movement.
 Mechanism :- in normal labor, head enters brim more commonly through available
transverse diameter (70%) & less commonly through one of the oblique diameters.
 Position – occipitolateral or oblique occipitoanterior. LOA common than ROA.
 Engaging antero-posterior diameter of head – SOB 9.5cm or SOF 10cm (in slight
deflexion). Engaging transverse diameter is biparietal 9.5cm.

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STEPS OF LABOR FOR OL POSITION
 Engagement
 Descent
 Flexion
 Internal rotation
 Crowning
 Extension
 Restitution
 External rotation of head & internal rotation of shoulders
 Expulsion of the trunk

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ENGAGEMENT
 Primigravidae – before onset of labor. Multiparae – late first stage with rupture of
membranes.
 Due to lateral inclination of head, sagittal suture (SS) does not strictly correspond to
transverse diameter of inlet – deflected anteriorly towards symphysis pubis (SP) or
posteriorly towards sacral promontory (SPr).
 This Deflection – asynclitism. Mild degree is common. Severe degree asynclitism –
cephalopelvic disproportion.
 If SS lie ant., SP; post. Parietal Bone (PB) presenting part - post. Asynclitism or post.
Parietal presentation. Common in prmigravidae. Post. lateral flexion to glide ant. PB.
 If SS lie post., SPr.; ant. PB presenting part – ant. Asynclitism or ant. Parietal
presentation. Common in multiparae. Ant. lateral flexion to glide post. PB.

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Head brim relation prior to engagement—
(A) Anterior parietal presentation,
(B) Head in synclitism,
(C) Posterior parietal presentation
A B C
After flexion, head enters brim & synclitism occurs. In 25% cases.

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DESCENT
 If no undue bony or soft tissue obstruction – continuous process.
 Slow & insignificant in 1st stage & pronounced in 2nd stage & complete with expulsion of
fetus.
 Primigravidae – practically no descent in 1st stage with prior engagement. Multiparae –
starts with engagement
 Head expected to reach pelvic floor till cervix is fully dilated
 Factors:-
 Uterine contraction & retraction
 Bearing down efforts
 Straightening of fetal ovoid specially after rupture of the membranes.

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FLEXION
 Some degree noticeable at beginning of the labor. Complete flexion –
uncommon.
 Factors:- due to resistance offered by :-
 Unfolding cervix, Walls of the pelvis and Pelvic floor
 Precedes internal rotation or coincides.
 Necessary for descent – reduces the shape and size of the plane of
advancing diameter of the head
 Two arm lever theory:- fulcrum & 2 arms:-
 Fulcrum – occipito-allantoid joint of head (joins the head to body)
 Short arm – condyles to occipital protuberance & Long arm – condyles to chin
 On Resistance – short descend & long arm ascend = flexion

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Lever action producing flexion of the
head reducing the engaging diameter of the head from—
(A) Occipitofrontal to (B) Suboccipito bregmatic

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INTERNAL ROTATION OF THE HEAD
 No further descent without it. Descent accompanies internal rotation.
 Theories:-
 Slope of pelvic floor :- 2 halves of levator ani form a gutter, direction of fibres is
backwards & towards midline.
 During each contraction, head, occiput, in well flexed position, stretches levator ani.
After contraction, elastic recoiling of levator ani, brings occiput forward toward
midline. Repeats until occiput placed anteriorly. K/a rotation by law of pelvic floor
(Hart’s rule).
 Pelvic shape :- forward inclination of side walls of cavity, narrow bispinous diameter,
long anteroposterior diameter of outlet – put long axis of head in maximum available
diameter, AP diameter.
 Law of unequal flexibility :- internal rotation due to inequalities in flexibility of
component parts of fetus.

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Cont..
 OL position – ant. rotation by 2/8th of a circle of occiput
 Oblique ant position – 1/8th of a circle forward, place occiput behind symphysis pubis.
 Prerequisites for Ant. Internal rotation :- well flexed head, efficient uterine contraction,
tone of levator ani muscles & favourable shape of midpelvic plane.
 Level of internal rotation varies – cervix, favourable but less frequent. Mostly at pelvic
floor.
 Torsion of neck :- inevitable phenomenon during internal rotation.
 If the shoulders remain in AP diameter, neck has to sustain torsion of 2/8th of a circle
which fails – some amount of rotation of shoulders in same direction of 1/8th of circle –
place shoulder in oblique diameter and 1/8th left behind.
 Shoulders occupy left oblique diameter in LOL position & right in ROL
 Oblique occipitoanterior position – no movement of shoulders as neck sustains
only1/8th of a circle.

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CROWNING
 After internal rotation, further descent occurs until subpcciput
lies under pubic arch.
 Maximum diameter of head – biparietal diameter stretches vulval
outlet without recession of head even after contraction is over –
k/a crowning of head.

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EXTENSION
 Delivery of the head – extension through “couple of force” theory.
 Driving force pushes head – downward direction
 Resistance by pelvic floor – upward & forward direction
 Downward & Upward neutralize. Forward thrust help in extension.
 Successive parts of the head to be born through stretched vulval
outlet – vertex, brow & face.
 Following release of the chin through stretched perineum, head drops
down, brings chin close to maternal anal opening.

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Lateral view showing mechanism of labor in
left occipitolateral position
(A and B) Posterior
parietal presentation,
posterior lateral flexion
of the head and
engagement.
(C and D) Internal
rotation of the head with
movement of the
shoulders;
descent and delivery of
the head by extension

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RESTITUTION
 Visible passive movement of the head due to untwisting of the
neck sustained during internal rotation.
 Head rotates through 1/8th of circle in direction opposite to
internal rotation.
 Occiput points to maternal thigh corresponding to original lay.

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INTERNAL ROTATION OF SHOULDERS
& EXTERNAL ROTATION OF THE HEAD
 Movement of rotation of the head visible
externally due to internal rotation of the
shoulders.
 Ant. Shoulder rotates toward symphysis
pubis from oblique diameter – external
rotation of head by 1/8th in same direction of
restitution.
 Now shoulders in AP diameter. Occiput
points maternal thigh corresponding to
original lay during engagement.

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BIRTH OF SHOULDERS & TRUNK
 After shoulders in AP diameter – further descent – anterior shoulder
escapes below symphysis pubis first.
 Lateral flexion of spine – post. shoulder sweeps over perineum.
 Lateral flexion - Rest of the trunk expelled out
Anterior shoulder Posterior shoulder

Postition of fetus and Mechanism of labor

Postition of fetus and Mechanism of labor

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