Mechanism of labor in right occipito posterior position
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This document discusses the mechanism of labor for babies in the right occipito-posterior (ROP) position. It describes two main mechanisms - the long arc rotation of 135 degrees and the short arc rotation of 45 degrees. For long arc rotation, the baby's head enters the pelvis with the sagittal suture in the right oblique diameter and rotates 135 degrees anteriorly. For short arc rotation, the head also enters in the right oblique diameter but only rotates 45 degrees anteriorly, placing the occiput in the hollow of the sacrum. Both mechanisms can result in arrest if full rotation does not occur. The OP position is associated with prolonged labor stages and higher rates of operative delivery and maternal trauma
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Mechanism of labor in right occipito posterior position
- 1. Mechanism of labor in right occipito-posterior position By: Gautam Hariish and Jenadia Naidoo
- 2. Occiput Posterior Position • Occiput placed posteriorly over sacroiliac joint or directly over sacrum. • Fetal face towards maternal symphysis pubis or anterior segment of pelvis • More common in android and anthropoid pelvis
- 3. Mechanism of labor in ROP • There are 2 mechanisms: –Anterior rotation: Long arc (135°) –Posterior rotation: Short arc (45°)
- 4. Mechanism in long arc (135°) rotation Descent •Head enters the pelvis with sagittal suture in the right oblique diameter. • Descent continues throughout labor unless any obstruction is encountered.
- 5. Mechanism in long arc (135°) rotation(continued) Engagement • Engaging transverse diameter of the head is biparietal diameter. • AP diameter is either: - Suboccipitofrontal due to incomplete flexion -Occipitofrontal due to marked deflexion
- 6. Mechanism in long arc (135°) rotation(continued) Flexion • Imperfect and often not complete till head reaches the pelvic floor. • Partial flexion and the resulting larger diameter of the presenting part contributes to prolonged labor.
- 7. Mechanism in long arc (135°) rotation(continued) Internal rotation • Occiput is the leading part and rotates 135° anteriorly and lie behind the symphysis pubis. • Neck cannot sustain such torsion, the shoulders rotate about 90° to occupy the right oblique diameter. (lag of 45°)
- 8. Mechanism in long arc (135°) rotation(continued) Extension • Head delivers by extension • Nape of neck pivots under the sub pubic angle and the head is born in extension BREGMA FOREHEAD NOSE CHIN (Pass over the perineum in this order.)
- 9. Mechanism in long arc (135°) rotation(continued) Restitution • After delivery of the head , it restitutes to the right from OA ROA • The extent depends on the degree that shoulders have followed the head during internal rotation. • In most cases, the shoulders lag by 45°, so restitution is by 45°.
- 10. Mechanism in long arc (135°) rotation(continued) External rotation • The anterior shoulder strikes the pelvic floor and rotates 45° towards the symphysis pubis. • The bisacromial diameter of the shoulders is in the AP diameter of the pelvic outlet. • The birth of shoulders and trunk by lateral flexion.
- 11. Mechanism in short arc (45°) rotation Descent • Head enters pelvic inlet with sagittal suture in the right oblique diameter of the pelvis. • Descend continuous throughout labor.
- 12. Mechanism in short arc (45°) rotation (continued) Flexion • It is imperfect resulting in longer presenting diameter.
- 13. Mechanism in short arc (45°) rotation (continued) Internal rotation • As sinciput hits pelvic floor, it moves anterior. • The occiput turns posterior by 45° ROP → OP into hollow of sacrum. • Sagittal suture is in AP diameter of pelvis and bregma is behind pubis.
- 14. Mechanism in short arc (45°) rotation (continued) Birth of head by combination of flexion and extension Good flexion Incomplete flexion • The area under the bregma pivots under the symphysis • The root of nose pivots under the symphysis • Presenting diameter: Suboccipitofrontal(10.5cm) • Presenting diameter: Occipitofrontal diameter(11.5 cm) • Bregma, vertex, small fontanelles and occiput are born by further flexion • Forehead, bregma, vertex and occiput are born by flexion • Less traumatic • More traumatic
- 15. Mechanism in short arc (45°) rotation (continued) Restitution • The head moves 45° in the direction opposite to that of internal rotation. • Face look towards mother’s left thigh in ROP.
- 16. Mechanism in short arc (45°) rotation (continued) External rotation •Anterior shoulder strikes pelvic floor and rotates 45° towards symphysis pubis. •This brings the bisacromial diameter to AP diameter of the pelvis. •Results in external rotation of head 45° to the right. •Birth of shoulders and trunk by lateral flexion.
- 17. DEEP TRANSVERSE ARREST • Incomplete forward rotation. • Occiput rotates through 45° forwards. • Sagittal suture comes to lie in bispinous diameter. • Further rotation unlikely. OBLIQUE POSTERIOR ARREST • Non rotation. • Both sinciput and occiput, touch the pelvic floor simultaneously due to deflexion of head. • Sagittal suture lies in oblique diameter. Non Rotation or Malrotation
- 18. OCCIPITOSACRAL ARREST • Due to extreme deflexion. • Sinciput touches pelvic floor & rotates anterior. • This puts occiput in sacral hollow. • Direct occipitoposterior or persistent occipito posterior position. • Can deliver by face to pubis but arrest can occur Non Rotation or Malrotation (continued)
- 19. Consequences of OP position • Prolonged 1st and 2nd stages of labor. • High incidence of operative delivery. • Increased chance of intervention such as ARM and oxytocin augmentation due to prolonged labor. • Maternal trauma such as anal sphincter injury due to operative delivery.
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