Mechanism of Labor

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MECHANISM OF NORMAL LABOR
By: Mrs. Ashwini Khope
Lecturer, DES College of Nursing,
Shivajinagar, Pune
Mechanism of normal labor is the most important aspect to be studied in midwifery. It not only guides the actions of be performed while conducting a normal delivery but its study also ensures a safe and uneventful normal vaginal delivery. Most of us in our clinical experience must have observed how a normal delivery is conducted. But very few actually realize that in the hurry of conducting the delivery we fail to wait and observe the movements that take place during the delivery. A good midwife will always aid in conduction of a vaginal delivery and not hastily conduct it without knowing about the mechanism that is occurring inside the pelvis. Hence it is very essential to learn about mechanism of normal labor, its principles and steps to distinguish it from an abnormal one 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 labor.1 Principles

The principles are common to all mechanisms
1. Descent occurs throughout
2. 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

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...
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