Mechanism of normal labour
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Normal labor involves several stages of fetal positioning and movement through the female pelvis. The fetal skull flexes to reduce its diameter for descent through the birth canal. As labor progresses, the baby engages and the posterior parietal bone glides past the sacral promontory. During crowning, the biparietal diameter stretches the vulval outlet without recession. The shoulders then rotate and the occiput faces the pubic symphysis. Birth of the shoulders and trunk follows, with the posterior shoulder sweeping the perineum and the trunk expelled in lateral flexion. External rotation and restitution position the head for extension and release of the chin through the perineum.
Mechanism of normal labour
- 4. Fetal Skull
- 7. Flexion Full flexion of head reduced the advancing diameter of the head. Descent It is a continuous process. Engagement Baby lies in Flexion. Posterior parietal bone glides past the sacral promontory in anterior parietal presentation.
- 8. Crowning Maximum diameter of the head (biparietal diameter) stretches the vulval outlet without any recession of the head even after the contraction is over. Torsion of neck Rotation of shoulders in the same direction to the extent of 1/8th of the circle. Shoulders along oblique diameter. Internal Rotation Anterior rotation by 2/8th of a circle of the occiput, in occipito-lateral position. The rotation will be 1/8th in oblique anterior position. Occiput faces Pubic Symphysis.
- 9. Birth of shoulders and trunk Posterior shoulder sweeps the perineum. Trunk expelled in lateral flexion. External Rotation Rotation of the head through 1/8th of a circle in same direction as restitution. Shoulders in antero-posterior diameter. Restitution Rotating the head through 1/8th of a circle in direction opposite to the internal rotation. The occiput points to the maternal thigh of the corresponding side to which it originally lay. Extension Release of the chin through the anterior margin of the stretched perineum, the head drops down, bringing the chin in close proximity to the maternal anal opening.