labour

Mechanism of labor

Dr.Hala A.G.AL-Rawi

Labour

can be defined as the process by which regular painful contractions bring about effacement and dilatation of the cervix and descent of the presenting part, ultimately leading to expulsion of the fetus and the placenta from the mother

Fetal lie:

The relation of the long axis of the fetus to that of the mother.
1.longitudinal lie:99%of labors at term.
2.transverse lie:multiparity,placenta previa hydramnios,uterine anomalies.
3.oblique lie :maternal and fetal axis cross at 45 angle most are unstable and become longitudinal or transverse at labor.

Fetal lie

ATTITUDE OR POSTURE

• Is the relation of fetal parts to each others.
• In later months posture of the fetus folded on itself to accommodate the shape of the uterus (flexed head, thighs, knees &feet ,the arms crossed over the chest)
• Change from this flexed attitude can cause abnormal presentations

FETAL POSITION

The relation of a chosen point of the fetal presenting part to the Rt or Lt side of maternal birth canal
The chosen point:
Vertex presentation-occiput
Face presentation-mentum
Breech presentation-sacrum

Each presentation has two positions Rt or Lt

Each position has 3 varieties :
Antreior, transverse, posterior

MECHANISM OF LABOUR

This refers to the series of changes in position and
attitude that the fetus undergoes during its passagethrough the birth canal.

It is described here for the vertex presentation and the gynaecoid pelvis.

The relation of the fetal head and body to the maternal
pelvis changes as the fetus descends through the pelvis.
This is essential so that the optimal diameters of thefetal skull are present at each stage of the descent

MECHANISM OF LABOUR WITH OCCIPUT PRESENTATIONSTHE CARDINAL MOVEMENTS OF LABOUR

1-ENGAGEMENTThe widest part of the presenting part haspassed through the pelvic inletIt may occur in the last few weeks of pregnancy or only in labour especially in multipara


The number of fifths of the fetal head palpable abdominally is often used to describe weather engagement has taken place.
If more than two-fifths of the fetal head is palpable abdominally,the head is not yet engaged

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 during the first stage and first phase of the second stageof labor
In the active phase of the second stage of labour ,descent of the fetus is helped by voluntary use of abdominal musculature and the valsalva manoeuvre(pushing)





The descending head meets resistance of pelvic floor, Cervix & walls of the pelvis   flexion


The shorter suboccipito-begmatic is substituted for the longer occipito-frontal

• 3-flexion

• Lever action producing ftexion of the head; conversion from occipitofrontal to suboccipitobregmatic diameter typically reduces the anteroposterior diameter

4-INTERNAL ROTATION

Turning of the head from the OT position  anteriorly towards the symphysis pubis ie. Occiput moves from transverse to ant 45º if the head is well flexed (The levator ani muscles form a V shaped sling that tend to rotate the vertex anteriorly

Less commonly OT  posteriorly towards the sacrum 135º

It is not accomplished till the head has reached the spines

5-EXTENSION

Folloing compietion of internal rotation ,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 pubis

Crowning  the largest diameter of the fetal head is encircled by the vulvar ring

The head is born by further extension as the occiput,escapes from underneath thesymphysis pubes while bregma (ant.fontanelle), forehead, nose, mouth & chin pass successively over the perineum

RESTITUTION

After delivery of the head,the occiput is directly anterior.as soon as it escapes from the vulva,the head aligns itself with the shoulders,which have entered the pelvis in the oblique position.
The slight rotation of the occiput through one –eighth of a circle is called

6-EXTERNAL ROTATION

In order to be delivered,shoulders have to rotate into the the direct AP plane (the widest diameter at the outlet).When this occurs,the occiput rotates through a further one-eight of a circle to the transverse position.
This is called external rotation.

Delivery of shoulders and fetal body

When restitution & external rotation have occurred,the shoulders will be in the AP position.
The anterior shoulder is under the symphysis pubis and delivers first,and the posterior shoulder deliveres subsequently.

Although this process may occur without assistance,lateral traction is often exerted by gently pulling the fetal head in a downward direction to help release the anterior shoulder from beneath the pubic symphysis
The rest of the body delivered easily.