MECHANISM OF LABOUR

 It is the series of movement by fetus herd in its birth passage during its journey through the pelvis

(1) Engagement:-

Fetal presenting part has passed through pelvic inlet at the level of the ischeal spine (station- 0) 

In nulliparous engagement occurs before onset of labour (up t 2 week before)

In multipara engagement usual occurs with onset of labour because tone is more lax.

(2) Descent:-

Descent occurs throughout labour & is the down ward movement of the fetus (It begins before the onset of labour)

By the force of amniotic fluid during second stage of labour bearing down increases in to abdominal pressure these factor’s causes descent:-

(i) Floating:- Fetal presenting part is not engaged in pelvic inlet

(ii) Fixed- Fetal presenting part has entered pelvis (dipping)

(iii) Engagement:- Fetal presenting part (Usually BPD of fetal head) has passed through pelvic inlet

(iv) Station:- Location of the presenting part in relation to the level of ischeal spine. 

(v) Station -1, -2, -3, -4 occur when the presenting part is 1, 2, 3, 4cm above the ischeal spine.

(vi) O-station at the isheal spine

(vii) Station +1, +2, +3, +4, occur when the presenting part is 1, 2, 3, 4 below the level of Ischeal spine.

(viii) A station +4 Indicate that the presenting part is on the pelvic floor (On the perineum)

(3) Flexion:-

Resistance to descent by the pelvic floor causes head to fix so the chin is close to the chest.

This causes the smallest diameter of head (Sub-occipito bregmatic) presenting through the canal

This puts the posterior fontanels at almost the center of the cervix (easily palpable on vaginal examination)

Flexion begins at the pelvic brim a continuous until the fetal head reaches the pelvic floor.

(4) Internal Rotation of head:-

It is a movement of great Importance without which there will be no further descent

Due to contraction Leading part is pushed down ward on the pelvic floor & the resistance of muscular diaphragm brings about rotation.

Resistance of this an important determinant of Rotation

In a well flexed vertex presentation the occipital leads and meets the pelvic floor first rotate anteriorly though V8  of circle toward symphysis pubis

LOA/ROA position- Due to this the slight twist

(V8) in the neck of fetus is remaining

LOT/ROT:- First Y8 rotate head then 1/8 rotate body & than 1/8 rotate head means = 2/8 rotate head & 1/8 rotate body

LOP/ROP- First 1/8 rotate head than 1/8 rotate body & then 1/8 rotate head than 1/8 rotate body than 1/8 rotate head means = 3/8 rotate head & 2/8 rotate body.

(5) Crowning:- The maximum diameter of the head (BPD) stretches the vulval  out let without any secession of the head even after the contraction is over called crowning of head,

After the internal rotation of the head further descent occur & the occiput slips beneath the supra pubic arch & crowning occurs. 

(6) Extension:-

Delivery of head takes place through or by extension through couple of force theory.

The driving force pushes the head in a down word direction while the pelvic floor offers a resistance in the upward > forward dissection 

The down ward & upward forces neutralize & remaining forward though helping in extension

Due to sinciput face & chin which sweep the perineum & born by a movement of extension.

Immediately following the release of the chin through the anterior margin of the stretched perineum the head drops bringing the chin is close proximity to the maternal opening.

(7) Restitution:-

It is simple the fetal head returning to its normal relationship with the shoulder.

It is visible passive movement for the untwisting of the neck resets sustained during internal rotation 

Head rotate 1/8th of a circle in the opposite dissection of the internal rotation.

(8) External Rotation or Internal rotation of shoulder:-

After the restitution the external rotation occurs as the body rotates so that the shoulder is in the anterior-posterior diameter of the pelvis

The anterior shoulder is the first to reach the lavatory any muscles & therefore rotates 1/8 interiorly to lie under the symphysis pubis.

This movement can be clearly seen as the hard turns to the same time.

It occurs in the same direction as restitution the occiput of the fetal head now lies laterally shoulder is lie the anterior-posterior diameter.

(9) Delivery of shoulder:-

After delivery of the infant head a internal rotation of the shoulder the further descent takes place until the anterior shoulder escape below the symphysis pubic first.

This posterior shoulder delivers by the movement of lateral flexion.

(10) Expulsion of the Trunk:- After delivery of shoulder the rest of the body expelled out by lateral flexion.