OBJECTIVES
General Objectives :
By The End Of The Class Thee Students Will Be Able To Gain Indepth
Knowledge Regarding Mechanism Of Labour.
Specific Objectives :
By The End Of The Class Presentation The Group Will Be Able To
Define Mechanism Of Labour
Expalin About Mechanism Of Labor
Expalin About Cardinal Movements Of The Mechanism Of Labour
Summary Of Mechanism Of Labour
MECHANISM OF NORMAL LABOR
Definition
The Series Of Movements That Occurs On The Head In The Process Of
Adaptation During Its Journey Through The Pelvis Is Called Mechanism Of
Labor
It Should Be Borne In Mind That While The Principal 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 .
Mechanism ;
In Normal Labor,The Head Enters The Brim More Commonly Through
The Available Transeverse Diameter 70 Persent And To A Lesser Extent
Through One Of The Obliqe Diameters . Accordingly, The Position Is Little
More Common Than Right Occipito Anterior As The Left Oblique Diameter Is
Enchroached By The Rectum.
The Engaging Anterioposterior Diameter Of The Head Is Either Sub
Occipito Bregmatic 9.5cm Or In Slight Deflexion – The Occipitofrotal 10cm
The Engaging Transever Diameter Is Biparital 9.5cm .As The Occipito
Lateral Position Is The Most Common , The Mechanism Of Labor In Such
Position Will Be Described .
The Principal Movements Are The ;
Engagement
Desent
Flexion
Internal Rotation
Crowning
Extension
Restitution
External Rotation
Expulsion Of The Trunk.
All Though The Various Movements Are Described Separately But In
Reality , Yhe Movements At Least Some May Be Going On Simultaniously.
Engagement :
Head Brim Relation Priar To The Engagement As Revealed By Imaging
Studies Shows That Due To Lateral Inclination Of The Head , The Sagital
Suture Does Not Strctly Correspond With The Available Transeverse Diameter
Of The Client .
Instead , It Is Either Deflected Anteriorly Towards The Symphysis Pubis Or
Posteriorly Towords The Sacral Promontary .Such Deflexion Of The Head In
Relation To The Pelvis Is Called Asynclitism.
When The Sagital Suture Lies Anteriorly , The Posterior Parietal Bone
Becomes The Leading Presenting Part Is Called Postirior Asynclitism Or
Posterior Parital Presentation . This Is More Frequently Found In Primigravida
Because Of Good Uterine Tone And Tight Abdominal Wall.
In Others The Sagital Suture Lies More Posteriorly With The Result That
The Anterior Parital Bone Becomes The Leading Presenting Part And Is Then
Called Anterior Parital Presentation (Or) Anterior Asynclitism.It Is More
Commonly Found In The Multipara.
“Mild Degrees Of Asynclitism Are Common But Severe Degrees
Indicate Cephalopelvic Disproportion.
Posterior Lateral Flexion Of The Head Occurs To Glide The Anterior
Parietal Bone Past The Symphysis Pubis In Posterior Parietal
Presentation.Lateral Flexion In The Reserve Direction Occurs To Guide The
Posterior Parietal Bone Past The Sacral Promontary In Anterior Parietal
Presentation.
After This Movement Which Occurs Early In Labour,Not Onlt Thehead
Enters The Brim But Also Synclitism Occurs. However In About 25% Of Cases
The Head Enters The Brim In Synclitism I,E The Sagital Suture Corresponds
To The Diameter Of Engagement.
In Primigravidae,Engagement Occurs In A Significance Number Of
Cases Before The Onset Of Labor While In Multiparae.The Same May Occurs
In Late First Stage With Rupture Of The Membranes.
Descent :
Provided There Is No Bony Or Soft Tissue Obstruction,Descent Is A
Continueous Process.It Is Slow Or Significance In First Stage But Pronounced
In Second Stage.
It Is Completed With The Expulsion Of The Fetus. In Primigravida With
Prior Engagment In First Stage While In Multipara Descent Starts With
Engagement.
Head Is Expected To Reach The Pelvic Floor By The Cervix Is Fully
Dilated.
Factors Facilitating Descent Are,
Uterine Contraction And Retraction,
Bearing Down Efforts And
Straightening Of The Avoid Fetal Especially After Rupture Of The
Membranes.
Flexion :
While Some Degree Of Flexion Of The Head Is Noticeable At The Beginning
Of Labor But Completed Flexion Is Rather Uncommon.As The Head Meets
Resistance Of The Birth Canal During Descent Full Flexion Is Achieved.
Thus If The Pelvis Adequet,Flexion Is Achieved Either Due To The
Resistance Offered By Unfolding Cervix The Walls Of The Pelvis Or By The
Pelvic Floor.
It Has Been Seen That Flexion Proceds Internal Rotation Or At Least
Concide With It.Flexion Is Essential For Descent Since It Reduces The Shape
And Size Of The Plane Of The Advancing Diameter Of The Head.
Flexion Is Expalined By The Two Arm Lever Theory:
The Fulerum Respected By The Occipitoallantoid Joint Of The Head The Short
Arm Extends From The Condyles To The Occipital Protuberence And The
Long Arm Extends From Condyles To The Chin. When Resistence Is
Encountered By Oridinary Law Of Mechanics The Short Arm Descends And
The Long And Ascends Resulting In Flexion Of The Head.
Internal Rotation :
It Is A Movement Of Great Importance Without Which There Will Be No
Further Descent The Mechanism Of Internal Rotation Is Very Complex
Although Easy To Describe.
The Theories Which Explain The Anterior Rotation Of The Occiput.
Slope Of Pelvic Floor :Two Halves Of Levator Ani Form A Gutter And Viewed
From Above The Direction Of The Fibers Is Backward And Towards The
Midline.
Thus During Each Contraction The Head Occiput In Particular In Well
Flexed Position Stretches The Levator Ani Particularly That Half Which Is
Relation To The Occiput.
After The Contraction Passes Off,Elastic Recoil Of The Levator Ani Occurs
Bringing The Occiput Forward Toward The Midline.The Process Is Repeated
Until The Occiput Is Placed Anteriorly.This Is Called Rotation By The Law Of
Pelvic Floor.(Harts Rule).
Pelvic Shape ;
Fporward Inclination Of The Side Walls Of The Cavity , Narrow Bispinous
Diameter Of The Outlet Result In Putting The Long Axis Of The Head To
Accommodate In The Maximum Available Diameter I.E. Anterioposterior
Diameter Of The Outlet Leaving Behind The Smallest Bispinous Diameter.
Law Of Unequal Flexibility ;
The Internal Rotation Is Primarily Due To Inequalities In The Flexibility Of The
Component Parts Of The Fetus .
In Occipito Lateral Position, There Will Be Anterior Rotation By Two – Eight
Of A Circle Of The Occiput . Where As In Oblique Anterior Position, Rotation
Will Be One – Eight Of A Circle Forward , Placing The Occiput Behind The
Symphysis Pubis . There Is Always An Accompanying Movements Of Desent
With Internal Rotation.
Thus, Prerequisites Of Anterior Internal Rotation Of The Head Are Well –
Flexed Head , Efficient Uterine Contraction, Favorable Shape At The Mid
Pelvic Plane, And Tone Of The Levator Ani Muscles.
The Level At Which Internal Rotation Occurs Is Variable. Rotation In The
Cervix Although Favaroble Is Less Frequent Occurance . In Majority Of Cases ,
Rotation Occurs At The Pelvic Floor. Rarely, It Occurs As Late As Crowning
Of The Head .
Torsion Of The Neck ;
Torsion Of The Neck Is An Invitable Phenomenon During Internal Rotation
Of The Head. If The Shoulders Remain In The Antewrio Posterior Diameter,
The Neck Has To Sustaind A Torsion Of Two – Eight Of Circle Corresponding
With The Same Degree Of Anterior Rotation Of The Occiput .
But The Neck Fails To With Stand Such Major Degree Of Torsion And As
Such There Will Be Some Amount Of Simultaniously Rotation Of The
Shoulders In The Same Direction To The Extent Of One- Eight Of A Circle
Planing The Shoulders To Lie In The Oblique Diameter With One – Eight Of
Torsion Still Left Behind.
Thus ,The Shoulders Move To Occupy The Left Oblique Diameter In Left
Occipito Lateral Position. In Oblique Occipito Anterior Position. There Is No
Movement Of The Shoulder From The Oblique Diameter As The Neck Sustains
A Torsion Of Only One – Eight Of Circle.
Crowning ;
After Internal Rotation Of The Head, Further Desent Occurs Until The
Suboccipito Lies Underneath The Public Arch . At This Stage , The Maximum
Diameter Of The Head Streaches 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 Dawnward Direction While
The Pelvic Floor Offers A Resistance In The Upward And Forward Direction.
The Downward And Upward Forces Neutralize And Remaining Forward Thurst
Helping In Extension . The Successive Parts Of The Fetal Head To Be Born
Through The Streched Vulval Outlet Are Vertex , Brow And Face .
Immediately Following The Release Of The Chin Through The Anterior Margin
Of The Streched Perineum. The Head Drops Down ,Bring The Chin In Close
Proximity Yo The Maternal Anal Opening.
Restitution ;
It It The Visible Passive Movement Of The Head Due To Untwisting Of The
Neck Sustaind During Internal Rotation .Movement Of The Resti Tution Occurs
Rotating The Head Through One –Eighth 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 Movementvof The Rotation Ofd The Head Visible Externally Due
To Internal Rotation Of The Shoulders .
As The Anterior Shoulders Rotates Towards The Symphysis Pubis From The
Oblique Diameter, It Crries The Head Ina Movement Of External Rotation
Through One – Eightt Of The Circle In In The Same Direction As Restitution.
The Shoulders Now Lie In The Anteroposterior Diameter.
The Occiput Points Directly Toward The Maternal Thigh Corresponding To
The Side To Which It Originally Directed At The Time Of Engagement .
Birth Of The Shoulders And Trunk ;
After The Shoulders Are Positioned In Anteroposterior 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 Shoulders Sweeps Over The Perinium Rest Of The Trunk
Is Then Expelled Out By Lateral Flexion.
SUMMARY OF MECHANISM OF LABOR ;
Engagement
Increasing Flexion
Internal Rotation Of Occiput Anteriorly To 2/8 Of Circle Simultaniously
Rotation Of The Shoulders To 1/8 Of Circle .
Crowning
Delivery Of The Head By Extension
Restituition
External Rotation
Delivery Of The Shoulders And Trunk By Lateral Flexion.
SUMMARY OF MECHANISM OF LABOR ;
Engagement
Increasing Flexion
Internal Rotation Of Occiput Anteriorly To 2/8 Of Circle Simultaniously
Rotation Of The Shoulders To 1/8 Of Circle .
Crowning
Delivery Of The Head By Extension
Restituition
External Rotation
Delivery Of The Shoulders And Trunk By Lateral Flexion.
External Rotation
Delivery Of The Shoulders And Trunk By Lateral
Flexion.
