Mechanism of labour

MECHANISM OF LABOUR (NORMAL & ABNORMAL)

Lie, presentation, attitude, &position
FETAL LIEThe relation of the long axis of the fetus to that of the motherLongitudinal lie is found in 99% of labours at termPredisposing factors for transverse lie/oblique lie  multiparity, placenta previa, hydramnious, & uterine anomaliesFETAL PRESENTATIONThe presenting part is the portion of the body of the fetus that is foremost in the birth canalThe presenting part can be felt through the Cx on vaginal examinationLongitudinal lie  cephalic presentation breech presentationTransvrse lie  shoulder presentation

Lie, presentation, attitude, &position

CEPHALIC PRESENTATIONHead is flexed sharply  vertex / occiput presentationHead is extended sharply  face presentationPartially flexed  bregma presenting (sinciput presentation)Partially extended  brow presentationBREECH PRESENTATIONFrank breechComplete breechFootling breechATTITUDEPosture of the fetus folded on itself to accommodate the shape of the uterusFlexed head, thighs, knees &feetThe arms crossed over the chestFace presentation extended concave contour of the vertebral column

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(A) vertex
(B) sinciput
(C) brow
(D) face

Longitudinal lie. Frank breech presentation.

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Longitudinal lie. Complete breech presentation.

Longitudinal lie. Incomplete, or footling, breech presentation.

POSITION
The relation of an arbitrary chosen point of the fetal presenting part to the Rt or Lt side of the maternal birth canalThe chosen point Vertex presentation  occiputFace presentation  mentumBreech presentation SacrumEach presentation has two positions Rt or LtEach position has 3 varieties : Ant, transverse, post OA
OP
LOT
ROT
LOA
ROA
LOP
ROP



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LONGITUDINAL LIE VERTEX PRESENTATION
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Longitudinal lie. Vertex presentation
A. Right occiput posterior (ROP)
Right occiput transverse (ROT)

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Right occiput anterior (ROA).

FREQUENCY OF VARIOUS PRESENTATIONS & POSITIONS AT TERM

Vertex  96% 2/3 Lt 1/3 RtBreech  3.5%Face 0.3%Shoulder 0.4%

Longitudinal lie. Face presentation. Left and right anterior and ri posterior positions.

Rt mento-post
Rt mento-ant
Lt mento-ant


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Longitudinal lie Breech presentation LSP


Transverse lie. Right acromiodorsoposterior position (RADP). The shoulder of the fetus is to the mother's right, and the back is posterior.

MECHANISM OF LABOUR WITH OCCIPUT PRESENTATIONSTHE CARDINAL MOVEMENTS OF LABOUR

1-ENGAGEMENTThe greatest transverse diameter BPD passes through the pelvic inletIt may occur in the last few weeks of pregnancy or only in labour especially in multiparaThe fetus enters the pelvis in transverse or oblique diameterLOT  40%ROT 20%OP 20% ROP >LOP ROA / LOA 20%

THE CARDINAL MOVEMENTS OF LABOUR

Asynclitism The sagittal sutures of the head deflects ant towards the symphysis pubis or post towards the sacrum 2-DESCENT In nullipara engagement takes place before the onset of labour & further descent may not occur till the 2nd stage In multipara descent begins with engagement It is gradually progressive till the fetus is delivered It is affected by the uterine contractions & thinning of the lower segment

Anterior asynclitism Naegele's obliquity

Normal synclitism
Posterior asynclitism Litzmann's obliquity Ear presentation

3-FLEXION

The descending head meets resistance of pelvic floor, Cx & walls of the pelvis   flexionThe shorter suboccipito-begmatic is substituted for the longer occipito-frontal

Lever action producing ftexion of the head; conversion from occipitofrontal to suboccipitobregmatic diameter typically reduces the anteroposterior diameter from nearly 12- to 9.5 cm.


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4-INTERNAL ROTATION

Turning of the head from the OT position  anteriorly towards the symphysis pubis ie. Occiput moves from transverse to ant 45єLess commonly OT  posteriorly towards the sacrum 135єIt is not accomplished till the head has reached the spinesThe levator ani muscles form a V shaped sling that tend to rotate the vertex anteriorlyIt is completed by the time the head reaches the pelvic floor 2/3 or shortly after ј


EXTENSION
When the flexed head reaches the vulva it undergoes extension  the base of the occiput will be in direct contact with the inferior margin of the symphysis pubisCrowning  the largest diameter of the fetal head is encircled by the vulvar ringThe head is born by further extension as the occiput, bregma, forehead, nose, mouth & chin pass successively over the perineum

EXTERNAL ROTATIONRESTITUTION

After delivery of the head it returns to the position it occupied at engagement , the natural position relative to the shoulders (oblique position)Restitution Then the fetal body will rotate to bring one shoulder anterior behind the symphysis pubis ( biacromial diameter into the APD of the pelvic outlet) Restitution is followed by complete external rotation to transverse position (occiput lies to next to Lt maternal thigh) The ant shoulder slips under the pubis By lateral flexion of the fetal body the post shoulder will be delivered & the rest of the body will follow

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2.Engagement;descent, flexion
6. Restitution (external rotation)
3. Further descent, internal rotation
4. Complete rotation, beginning extension
Cardinal movements in the mechanism of labor and delivery, left occiput anterior position.

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OCCIPUT POSTERIOR POSITION

Mechanism of labour is identical to OT & anterior varieties The occiput rotate to the symphysis pubis through 135є instead of 90є or 45єIf rotation does not occur direct occiput post orPartial rotation transverse arrest


Mechanism of labor for right occiput posterior position, anterior rotation.
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