The part that leads and first meets the resistance of the pelvic floor will rotate until comes under the symphysis pubis 3. The part that escapes under the symphysis pubis will pivot around the pubic bone 4. During the mechanism, the fetus turns slightly to take advantage of the widest available space in each plane of the pelvis, i. e. transverse at the brim and antero-posterior at the outlet 2 Mechanism In normal labor, the head enters the brim more commonly through the available transverse diameter and to a lesser extent through one of the oblique diameters.
Accordingly, the position is either Occipito lateral or oblique Occipito anterior. The left Occipito anterior position is commoner than the right Occipito anterior position as the left Occipito diameter is encroached by the rectum. 1 The engaging antero-posterior diameter of the skull is suboccipito bregmatic (9. 5 cm) and the engaging transverse diameter is biparietal 9. 5 cm. 1 As the Occipito Anterior position is the commonest the mechanism of labor will be described for Left Occipito Anterior or Right Occipito Anterior position
When the fetus presents in left or right Occipito anterior position the way the fetus is normally situated can be described as follows: * The lie is longitudinal * The presentation is cephalic * The position is ROA or LOA * The attitude is one of flexion * The denominator is the occiput * The presenting part is the posterior part of the anterior parietal bone 2 Positional movements There are several basic positional movements which take place when the fetus is in a cephalic vertex presentation. These are as follows: 1. Engagement 2. Descent throughout 3. Flexion 4. Internal rotation of the head 5. Crowning 6.
Delivery of the head by extension 7. Restitution 8. Internal rotation of the shoulders 9. External rotation of the head 10. Birth of the shoulders and body by lateral flexion Although the mechanisms of labor are listed separately, some of these overlap or occur simultaneously (internal rotation of the shoulders and external rotation of the head) 1. Engagement Engagement takes place when the biparietal diameter of the fetal head has passed through the pelvic inlet. In primigravidae, it usually occurs during the latter weeks of pregnancy. In multigravidae, as the muscle tone is lax engagement may not occur until labor actually begins. 2. Descent Descent occurs throughout the mechanisms of labor and is therefore both requisite to and simultaneous with other mechanisms. Descent is a result of a number of forces including contractions and maternal pushing effort with contraction of her abdominal muscles 3. Flexion Flexion is essential to further descent. The pressure exerted down the fetal axis will be transmitted to the occiput. The effect is to increase flexion, which results in the substitution of the smaller suboccipito bregmatic diameter 9. 5 cm. the occiput becomes the leading part.
Flexion occurs when the fetal head meets the pelvic floor resistance which increases with descend. 4. Internal rotation of the head Internal rotation brings the anteroposterior diameter of the fetal head into alignment with the anteroposterior diameter of the maternal pelvis. This is accomplished by rotation of the occiput to the anterior portion of the maternal pelvis. In a well flexed vertex presentation, the occiput leads and meets the pelvic floor first and rotates anteriorly through one- eighth of a circle (45 degrees) this causes a slight twist in the neck of the fetus, as the head is no longer in direct alignment with the shoulders.
As the fetal head now lies in the widest (anteroposterior) diameter of the pelvic outlet, an easy escape is facilitated. The occiput slips under the pubic arch and crowning occurs when the head no longer recedes between contractions and the widest transverse diameter i. e. biparietal is born. 5. Crowning After internal rotation of the head further descent occurs until the subocciput lies underneath the pubic arch. At this stage, the maximum diameter of the head (biparietal diameter) stretches the vulval outlet without any recession of the head even after the contraction is over.
This is called “crowning of the head”. 6. Delivery of the head by extension Once crowning has occurred, fetal head can extend. The suboccipital region impinges under the symphysis pubis and acts as a pivotal point. The fetal head is now positioned so that further pressure from the contracting uterus and maternal pushing serves to further extend the head as the vaginal orifice opens. A couple of forces act on the fetal head. The driving force pushes the head in a downward direction while the pelvic floor offers a esistance in the upward and forward direction. The downward and upward forces neutralize and remaining forward thrust helps in extension. The successive parts of the fetal head to be born through the stretched vulval outlet are vertex, brow and face. 7. Restitution It is the visible passive movement of the head due to untwisting of the neck sustained during internal rotation. With restitution the occiput moves 45 degrees or one eighth of a circle towards the side from which it started and is again in alignment with the shoulders1 8.
Internal rotation of shoulders The anterior shoulder reaches the pelvic floor and rotates anteriorly by one eighth of a circle. The shoulders come to lie in the anteroposterior diameter of the pelvic outlet 9. External rotation of the head External rotation of the head occurs as the shoulders rotate 45 degrees internally causing the head to rotate another 45 degrees. The movement can be seen clearly as the head turns at the same time. External rotation occurs in the same direction as restitution. 10. Birth of the shoulders and body by lateral flexion
The anterior shoulder comes into view at the vaginal orifice, where it impinges under the symphysis pubis, while the posterior shoulder distends the perineum. This enables a smaller diameter to distend the vaginal orifice than if both the shoulders were born simultaneously. After the shoulders are delivered, the remainder of the body is born by lateral flexion. The mechanism of labor is different for different positions. Most of the movements in the mechanism occur simultaneously and at times may not be clearly visible during the delivery.
Hence the midwife should be very careful and cautious and keep in mind the steps of normal mechanism of labor while conducting delivery References 1. D. C Dutta, Textbook of Obstetrics, 6th Edition, New Central Book Agency, Calcutta 2004, Pp: 125-129 2. Annamma Jacob, A Comprehensive Textbook of Midwifery, 2nd Edition, Jaypee brothers Medical Publishers, New Delhi, 2009, Pp :187-190 3. www. wingkingskull. com cited on 31st January 2012 4. http://pregnancy. about. com/od/laborbasics/ss/fetalpositions_2. htm cited on 27th January 2012