Definition The series of movements that occur on the head in the process of adaptation, during its journey through the pelvis is called mechanism of labour. It should be in mind that while the principle 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. Mechanisms: In normal labour, vertex is presented either on LOA or ROA position. LOA is more common. In normal labour, the head enters the brim more commonly through the available transverse diameter (70%) which is longest diameter of the pelvis. The engaging antero-posterior diameter of the head is either suboccipito bregmatic 9.5cm or slight deflexion suboccipito-frontal 10 cm. The engaging tranverse diameter is biparital 9.5cm.The principal movements are:1. Engagement2. Descent3. Flexion4. Internal rotation of head5. Crowning of head6. Extension of head7. Restitution of the head8. Internal rotation of the shoulder 9. External rotation of the head occurs in same time 10. Lateral flexion of the body with expulsion of fetus. Mechanism of labour in vertex presentation (LOA) For normal mechanism, the fetus should be on following condition.i) Lie: should be longitudinal lie.ii) Attitude: Attitude must be in good flexioniii) Presentation: Should be vertex or cephalic presentation.iv) Position: Left occipito-anterior positionv) Denominator: Occiputvi) Presenting part: Posterior area of right parietal bone or sub occipital partvii) Engagement: Head should be well engaged in pelvic brim and cavity.viii) Station: Should be below the ischial spines. 1. Engagement In left occipito anterior, fetal head enters pelvic brim with occiput lying in relation to left iliopectineal eminence, sinciput at right sacroiliac joint and sagital suture lying on right oblique diameter of maternal pelvis. The engaging transverse diameter of fetal head is biparietal 9.5cm and the sub-occipito bregmatic diameter 9.5cm. When the sagital suture lies anteriorly, the posterior parietal bone becomes the leading presenting part and is called posterior parietal presentation. This is more frequently found in primigravidae because of good uterine tone and a tight abdominal wall. In others sagital suture lies more posteriorly with the result that the anterior parietal bone becomes the leading presenting part and is called anterior parietal presentation. It is more commonly found in multiparae. In primigravidae engagement occurs in a significant number of cases before onset of labour while in multiparae engagement occur in late first stage with rupture of the membranes.

2. Descent Descent is continuous process.It is slow or insignificant in first stage but pronounced in second stage. It is completed with expulsion of the fetus. In primigravidae, with prior engagement of the head, there is practically no descent in first stage, while in multiparae, descent starts with engagement. Head is expected to reach the pelvic floor by the time the cervix is fully dilated. Factors facilitating descent are:i. Uterine contraction and retractionii. Bearing down effortsiii. Straightenning of the fetal ovoid especially after rupture of membrane.3. Flexion While some degree of flexion of the head is noticeable at the begining of labour but complete flexion is rather uncommon. As the head meets the resistance of the birth canal during descent, full flexion is achieved. Thus if the pelvis is adequate, flexion is achieved either due to resistance offered by the unfolding cervix, the walls of the pelvis or by the pelvic floor. Flexion is essential for descent since it reduces the shape and sizes of the plane of the advancing diameter of the head. 4. Internal rotation of the head Internal rotation is a turning forward of whatever part of the fetus reaches the anterior, lateral half of the gutter shaped pelvic floor first. The resistance in the pelvic floor brings about rotation. In a well flexed vertex presentation, the occiput leads and meets the pelvic floor first and rotates anteriorly 1/8th of the circle. In a LOA position occiput rotates 450 from the left towards midline (from the left iliopectineal eminence to the symphysis pubis) where it can escape under the pubic arch and allow the sub-occipital region to pivot on the lower border of the symphysis pubis.Causes of internal rotation Slove of the pelvic floor: Two halves of the levator ani form a gutter shape, the direction of the fibers is backwards and towards the midline. Thus during each contraction the occiput stretches the levator ani and when the contraction passes off the elastic recoil of the levator ani occurs bringing the occiput forwards towards of the midline. The process is repeated until the occiput is placed anteriorly. Pelvic shape: Forward inclination of the side walls of the cavity, narrow bispinous diameter and long antero-posterior diameter of the outlet result in putting the long axis of the head to accommodate in the maximum available diameter i.e. antero-posterior diameter of the outlet. Law of unequal flexibility The internal rotation is primarily due to inequalities in the flexibility of the component parts of the uterus.

5. CrowningAfter the internal rotation of the head, further descent occurs until the sub-occiput lies underneath the pubic arch. At this stage the maximum diameters of the head (biparital diameter) stretches the vulval outlet without any recession of the head even after the contraction is over which is called crowning of the head.6. Extension of headDelivery of the head occurs by extension through couple of force theory. The driving force pushes the head in downwards direction while the pelvic floor offers a resistance in the upward and forward direction. The downward and upward forces neutralise and remaining forward thrust resulting in extension. During extension the sinciput, face and chin sweeps the perineum. The sun-occipito frontal diameter 10cm distends the vaginal outlet.7. RestitutionIt is the visible passive movement of the head to undo the twist in the neck, that took place during internal rotation of the head. The occiput restitutes 1/8th of the circle where it was before internal rotation of the head. The occiputs thus points to the maternal thigh of the corresponding side to which it originally lay.8. Internal rotation of the shouldersThis is a movement similar to internal rotation of the head. The shoulders in LOA are in the left oblique diameter of the pelvic cavity. The anterior shoulder reaches the right side of the pelvic floor and rotates forward bringing the shoulders into the antero-posterior diameter of the outlet.9. External rotation of headIt is the movement of rotation of the head visible externally due to internal rotation of the shoulders. As the anterior shoulder rotates towards the symphysis pubis from the oblique diameter, it carries the head in a movement of external rotation through 1/8th of a circle in the same direction as restitution. The shoulders now lie in the antero-posterior diameter. The occiput points directly towards matrnal thigh corresponding to the side to which it originally directed at the time of engagement.10. Lateral flexion of the bodyIt is a side-ways bending of the spine, which takes place while the body is being expelled. The shoulders are born sequentially, the anterior shoulder first. The anterior shoulder slips beneath the sub-pubic arch and the posterior shoulder passes over the perineum.