Hands-and-Knees Position and OASIS Risk
The hands-and-knees (all-fours) position is NOT associated with worse obstetric anal sphincter injuries compared to other positions; in fact, current evidence suggests it may be protective, though the data are limited.
Evidence Summary
Guideline Recommendations
The most recent 2022 French emergency medicine guidelines explicitly state that no childbirth position is demonstrably superior to another for preventing OASIS 1. These guidelines recommend that caregivers select the position in which both the patient and provider are most comfortable, while maintaining the ability to rapidly transition to supine position if needed for maneuvers like McRoberts 1.
Research Evidence on All-Fours Position
The available research actually suggests the all-fours position may be protective rather than harmful:
A large Swedish population study of 113,279 spontaneous vaginal births found that standing position had the lowest OASIS rates across all parities, while lithotomy position had the highest rates 2. The all-fours position was not specifically highlighted as high-risk in this comprehensive analysis.
A Norwegian cross-sectional study of 757 midwife-led births found that kneeling position was associated with significantly reduced OASIS risk (adjusted OR 0.15,95% CI 0.03-0.70), while supine position increased risk (adjusted OR 2.52,95% CI 1.04-4.90) 3.
A German retrospective cohort study of 7,832 deliveries found that while squatting on a low stool (OR 2.92) and kneeling positions (OR 2.14) were associated with increased AST risk compared to birth on bed, the kneeling position studied was specifically on a raised surface, not the hands-and-knees position 4.
Positions with Established Higher Risk
The evidence consistently identifies these positions as problematic:
Lithotomy position: Increased OASIS risk in both nulliparous (adjusted RR 1.17) and parous women (adjusted RR 1.66) compared to sitting 2
Supine position: Associated with increased OASIS risk (adjusted OR 2.52) 3
Squatting on birth stool: Increased risk in parous women (adjusted RR 2.16) 2
Clinical Implications
Position the patient based on comfort and clinical circumstances, but avoid lithotomy and supine positions when possible 2, 3. The hands-and-knees position should not be discouraged based on OASIS concerns, as it appears neutral to protective in the available evidence 3, 4.
Key Risk Factors to Address (Regardless of Position)
- Active pushing during crowning increases OASIS risk 3-fold (adjusted OR 3.10); instead, have the patient breathe the head out 3
- Maternal age, head circumference ≥35 cm, birth weight ≥4000 g, prolonged second stage, and oxytocin augmentation all increase risk 2
- Up to 35% of anal sphincter injuries are missed without digital rectal examination 5, 6, 7
Common Pitfall
Do not confuse "kneeling on a raised surface" with "hands-and-knees position" - these are biomechanically different positions with potentially different perineal mechanics 4. The hands-and-knees position distributes weight differently and may actually reduce posterior perineal pressure.