Occipito-Posterior (OP) Mechanism of Labour

Definition and Position

• Right Occipito-Posterior (ROP) - most common (occiput in right posterior quadrant)
• Left Occipito-Posterior (LOP)
• Direct Occipito-Posterior (DOP/OP) - occiput directly posterior at the sacrum

Incidence

• ~20% of fetuses present in OP at the onset of labour
• Only ~5% deliver in persistent OP ("persistent occipito-posterior")
• Thus, ~72-90% spontaneously rotate to an anterior position during labour

Mechanism of Labour in OP Position

Outcome 1: Long Rotation (135°) - Most Common Favourable Outcome

1. Engagement - The head engages in the pelvic brim in the OP position, often with the suboccipito-frontal diameter (10 cm) presenting, or the occipito-frontal diameter (11.5 cm) if deflexed. The engaging diameter is larger than in an OA position.
2. Descent - The head descends into the pelvic cavity. Uterine contractions and maternal bearing-down efforts drive descent.
3. Internal rotation (long rotation, 135°) - At the level of the ischial spines (pelvic floor), the occiput rotates anteriorly through 3/8 of a circle (135°) from posterior to anterior, so the occiput comes to lie under the symphysis pubis. This is the critical step. The head is now in the OA position.
4. Crowning and extension - The head extends at the perineum. The suboccipital region pivots under the symphysis pubis. The occiput, then the brow and face, are born over the perineum.
5. Restitution - The head rotates back by 45° to align with the fetal shoulders.
6. External rotation - A further 45° rotation occurs as the shoulders align with the AP diameter of the pelvis.
7. Birth of the trunk - Lateral flexion delivers the anterior shoulder under the pubic arch, then the posterior shoulder, then the trunk.

Outcome 2: Short Rotation (45°) - Persistent OP

1. Engagement - As above, in the OP position.
2. Descent - Into the pelvic cavity.
3. Internal rotation (short rotation, 45°) - The occiput rotates posteriorly (45°) to lie directly against the sacrum (DOP). The face now faces directly anteriorly under the symphysis.
4. Flexion then extension - The nape of the neck (suboccipital region) pivots against the coccyx (sacrococcygeal joint/perineum). The head undergoes:
   • Marked flexion first (chin on chest), delivering the occiput over the perineum
   • Then extension to deliver the face under the symphysis pubis
   • This is the reverse of the normal extension sequence
5. Restitution and external rotation - Head aligns with shoulders, then shoulders rotate to AP diameter.
6. Birth of trunk - As in normal mechanism.

Note: The face-to-pubis delivery is associated with a larger presenting diameter (occipito-frontal 11.5 cm vs suboccipito-bregmatic 9.5 cm in OA), so there is greater perineal trauma, deeper perineal tears, and longer second stage.

Outcome 3: Deep Transverse Arrest

Cardinal Movements in OP - Summary Table

Clinical Features of OP Labour

• Prolonged labour - especially first and second stages
• Persistent backache ("back labour") - occiput presses on maternal sacrum
• Premature urge to push - presenting part presses on rectum before full dilation
• Deflexed head - presents larger diameter (occipito-frontal 11.5 cm instead of 9.5 cm)
• Caput and moulding - more pronounced
• Higher rates of - augmentation, epidural use, instrumental delivery, perineal trauma, PPH, caesarean section

Causes / Predisposing Factors

• Anthropoid pelvis (long AP diameter) - favours OP engagement
• Android pelvis (narrow, funnel-shaped) - prevents anterior rotation
• Anterior placenta - fetus faces the placenta (goes OP)
• Deflexed head - prevents optimal engagement
• Epidural analgesia - relaxes pelvic floor muscles, reducing rotational forces
• Multiparity with lax abdominal muscles

Management

Key Measurements in OP Position