Biomechanical analyses of the efficacy of patterns of maternal effort on second-stage progress.

OBJECTIVE: To develop and use a biomechanical computer model to simulate the effect of varying the timing of voluntary maternal pushes during uterine contraction on second-stage labor duration.
METHODS: Published initial pelvic floor geometry was imported into technical computing software to build a simplified three-dimensional biomechanical model with six representative viscoelastic levator muscle bands interconnected by a hyperelastic iliococcygeal raphe. An incompressible sphere simulated the molded fetal head. Forces from uterine contraction and voluntary expulsive efforts were summed to push the model fetal head along the curve of Carus opposed by the resistance of the pelvic floor structures to stretch. Holding uterine maximal contraction force and push strength constant, pushes were timed before ("pre"), at ("peak"), and after ("post") maximal uterine contraction force. The effect of different combinations of pushes on second stage duration and the number of pushes required for delivery were evaluated.
RESULTS: Calculated second stage durations ranged from 57.5 minutes (triple or pre-peak-post pattern) to 75.8 minutes (prepush and postpush patterns). Delivery with the triple-push pattern required 59 voluntary pushes, while the peak-push pattern required 23 voluntary pushes, a 61% reduction. The corresponding reduction for the pre-and-peak-push pattern was 29%, the peak-and-post push pattern was 30%, the prepush pattern was 54%, and the postpush pattern was 56%.
CONCLUSION: Although the triple-push pattern resulted in a 16% shorter second stage, this came at the energetic expense of a 61% increase in the number of pushes required. LEVEL OF EVIDENCE: III.