Mechanism of Labor
The series of changes in position & attitude that the fetus undergoes during its passage through the birth canal. Engagement Descent Flexion Internal rotation Extension Restitution External rotation Birth of the shoulders& bodyTHE CARDINAL MOVEMENTS OF LABOUR
1-ENGAGEMENT The head enters the pelvis in transverse or oblique diameter Engagement occurs when the greatest transverse diameter (BPD) of the presenting part passes through the pelvic inlet ,it occurs in nulliparous before labour or only in labour in majority of multiparous women. Engagement can be determined by vaginal or abdominal examination, The number of fifths of the head palpable abdominally is used to detect engagement, if> than2/5th of fetal head is palpable abdominally ,the head is not yet engaged, an occiput at the ischial spines vaginally means engaged head (station zero)2-Descent Downward passage of presenting part through the pelvis. During the 1st stage& early 2nd stage of labour, descent of the fetus is secondary to uterine action. In the active phase of 2nd stage of labour ,fetal descent is helped by voluntary use of abdominal muscle & pushing. 3-Flexion Occurs as the head descends due to the shape of the bony pelvis and resistance of pelvic floor soft tissues, this allows smallest diameter of fetal head to pass through the pelvis.
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.
4. Internal Rotation: With well flexed head the occiput will be the leading point & on reaching the sloping gutter of the levator ani it will rotate about 45 degrees anteriorly to the midline under the symphysis. this brings the AP diameter of the head in line with the AP diameter of the pelvic outlet.
5-EXTENSION
When the flexed head reaches the perineum it undergoes extension the head escapes from underneath the symphysis pubis & distends the vulva.Crowning the largest diameter of the fetal head is encircled by the vulvar ringThe head is born by further extension as the bregma, forehead, nose, mouth & chin pass successively over the perineum6. Restitution
When the head is free of resistance, it aligns itself with the shoulders,which have entered the pelvis rotating through 1/8th of the circle.
7-External Rotation: The shoulders rotate into direct anterio-posterior orientation with further descent. This encourages the fetal head to return to its transverse position through a further 1/8th of a circle. The ant shoulder slips under the pubis by lateral downword flexion of the fetal body then post shoulder will be delivered & the rest of the body will follow