Mechanisms of Normal Labour

Mechanisms of Normal LabourPresented by:Nisha ChandDefinition 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, the head enters the brim more commonly through the available transverse diameter (70%) which is longest diameter of the pelvis.

The head which normally enter the brim in the tranverse or one of the oblique diameters undergoes rotation and also some changes in its attitude as it passes through the pelvic cavity and born from pelvic outlet in anterior-posterior diameter (longest diameter). Left occipito anterior is little common than right occipito-anterior as the left oblique dimeter is enroched by the rectum. The principal movement are:1. Engagement2. Descent3. Flexion

4. 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 time10. 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.Lie-should be longitudinal lie.Attitude: Attitude must be in good flexionPresentation: Should be vertex or cephalic presentation.Position: Left occipito-anterior positionDenominator: OcciputPresenting part: Posterior area of right parietal bone or sub occipital partEngagement: Head should be well engaged in pelvic brim and cavity. Station: Should be below the ischial spines.

1. EngagementIn 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. DescentDescent 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:Uterine contraction and retractionBearing down effortsStraightenning of the fetal ovoid specially after rupture of membrane.

3. FlexionWhile 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 headInternal rotation is a turning forwards of whatever part of the fetus reaches the anterior, lateral half of the gutter shaped pelvic floor first.