Comparing mechanical fetal response during descent, crowning, and restitution among deliveries with and without shoulder dystocia.

Objective: Previous computer simulations of shoulder dystocia (SD) explored the effect of SD itself on the mechanical response of the fetus. Our objective was to perform a mechanical simulation study to explore the variations in fetal response during routine, unilateral SD (USD), and bilateral SD (BSD) deliveries.

Study Design: Using a biofidelic birthing simulator, we performed 30 experiments mimicking passage of the fetus through the pelvis. For routine deliveries, we engaged the fetal head and allowed it to progress through cardinal movements using typical uterine contraction forces. Deliveries stopped when the head restituted externally to left occiput anterior (LOA) position. The identical procedure was repeated for USD deliveries, except we obstructed the anterior shoulder on the symphysis pubis; for BSD, the posterior shoulder was also impacted on the sacral promontory. For each delivery we continuously measured head rotation, brachial plexus (BP) stretch and neck extension, selecting peak values for analysis. Maximum rotation, BP stretch, and extension were compared among groups using analysis of variance, with P < .05 considered significant.

Results: Among routine, USD, and BSD deliveries, mean peak BP stretch varied between 10% and 21%, rotation varied between 70 degrees and 77 degrees, and extension varied between 6% and 18%. Greatest stretch occurred in the posterior BP during descent in non-SD deliveries, whereas anterior BP stretch, rotation, and extension were similar among the 3 types of deliveries.

Conclusion: Quantifiable mechanical response occurs in routine and SD deliveries. Posterior BP stretch is significantly longer for routine deliveries than either USD or BSD deliveries. By itself, shoulder dystocia does not pose additional risk of brachial plexus stretch over routine deliveries.