MECHANISM OF NORMAL LABOUR

Ans. is a, b  and c i.e. Flexion, Extension, Internal rotation, Descent and Asynclitisms 

Series of events that take place in the genital organs in an effort to expel the viable products of conception out of the womb through the vagina into the outer world is called as Labour.

Mechanism of normal labour is defined as the manner in which the fetus adjusts itself to pass through the parturient canal with minimal difficulty.

There are 8 cardinal movements of the head in normal labour.

• Every                –          Engagement (Synclitic or Asynclitic)
• Decent             –           Descent
• Female                          Flexion
• I                                    Internal rotation
• Choose to                     Crowning
• Employ            –            Extension
• Rises                             Restitution
• Extremely –                  External rotation Mnemonic : Every Decent Female 1 Choose to & Employ Rises Extremely.

Note :    Crowning is not taken as a separate cardinal movement of labour by most of the higher standard obs textbooks except ‘Nitta

Let us also understand what is engagement.

• Engagement is said to occur when the greatest transverse diameter of the presenting part (which in case of cephalic presentation is biparital° diameter9 has passed through the pelvic inlet.

Engagement is a sign that maternal pelvis is adequate for the particular head and that vaginal delivery is likely to occur.

Synctitic

When sagittal suture of the

head of fetus lies in the transverse diameter of pelvic inlet (midway between the pubic symphysis and the sacral promontory)

It occurs in 25% cases

Asynclitic

Anterior :

Sagittal suture is defected towards sacral promontory. Also k/a anterior parietal presentaion/Naegeles obliquity.

It occurs commonly in multipara

Posterior :

Sagittal suture is defected towards pubic symphysis. the posterior pari­etal bone thus becomes the lead­ing part. Also k/a Litzman obliquity/ parietal presentation

It occurs commonly in nulliparous women.

So in the question if instead of engagement, asynclitism or synclitism is given, it should also be taken as a cardinal movement.

Also know

• Most common presentation — cephalic (95%).
• Most common presenting part — vertex
• In normal labour, the head enters the pelvis more commonly through the transverse diameter° (75% cases) or oblique diameters (20% cases) (Ir^d most common) and AP diameter (5% cases) at the onset of labour°.
• The most common position (i.e. relation of occiput to quadrant of pelvis) is aft occipito-transverse.-
• The engaging diameter of head is Su^p occipito ureyr Val diarriu                (9.5 cm) or°

In slight deflexion is Sub occipito frontal (10 cm).°

Suboccipitofrontal

Beginning of labour

When the widest diameter of the presenting part has passed through the pelvic inlet

Biparietal diameter

The sequence of events occurring during normal labor are engagement, descent, increasing flexion, internal rotation, crowning, delivery of head by extension, restitution, external rotation and delivery of the shoulder and trunk by lateral flexion.

• During internal rotation, in occipito lateral position there is internal rotation of occiput anteriorly to 2/8th of the circle and simultaneous rotation of the shoulders to 1/8th of the circle.
• Crowning of head is said to occur when the biparietal diameter stretches the vulval outlet without any recession of the head even after the contraction is over.
• Delivery of head takes place by extension.
• Restitution: refers to the visible passive movement of the head due to untwisting of the neck sustained during internal rotation. 
• External rotation is the movement of rotation of head visible externally due to internal rotation of the shoulders.

Ref: Textbook of Obstetrics By D.C Dutta, 6th edn, page 126-8.

The diameter of engagement is the diameter of pelvic inlet in which head engages. Once engagement occurs, the vertex will be at the level of ischial spine and on abdominal palpation, either no part or only one-fifth of the head will be palpable.

Ref: Textbook of Obstetrics Sheila Balakrishnan 1st Ed, page 125-126

3/8^th of circle

Ans. C i.e. 10 cm

Ans. A i.e. 1 cm/ hour

Dilatation of the cervix at the rate of 1 cm/hour in primigravidae and 1.5 cm/hour in multigravidae beyond 3 cm dilatation (active phase of labor) is considered satisfactory