Poster 9: Effect of Voluntary Push Timing on the Duration of the Second Stage of Labor: Results from a Computer Simulation

OBJECTIVE: The objective of this study was to develop a dynamic biomechanical computer model to simulate the effect of varying the timing of the voluntary push, relative to peak uterine contraction (before, during, or after), on the duration of the second stage of labor. METHODS: The initial geometry of the female pelvic floor at the beginning of the second stage of labor was taken from a previous study (Lien et al. Obstet Gynecol. 2003; Fig. 1A). This geometry was projected onto the midsagittal plane so that a 2-dimensional biomechanical model could be implemented in Matlab (Fig. 1B). The 6 selected levator ani muscle bands were represented by a viscoelastic material interconnected by the hyperelastic iliococcygeal ligament (Fig. 1B), all tissue properties being taken from the literature. The molded fetal head was represented by an incompressible circle. To simulate the second stage of labor, the force from the uterine contraction and voluntary push were superposed and applied to push the model fetal head along the curve of Carus, opposed by the tensile pelvic floor structures without friction. The intensities and durations of uterine contraction and voluntary push were taken from the literature. To estimate the effect on vaginal birth, 3 voluntary push profiles were timed to occur at 25%, 50%, and 75% of the uterine contraction duration (Fig. 2).FIGURE 1: A, The initial geometry of pelvic floor tissues at the beginning of the second stage; B, biomechanical model showing pubic symphysis, fetal head, and pelvic floor.FIGURE 2: Time history of 3 different voluntary relative to peak uterine contraction.RESULTS: The bar graph (Fig. 3) shows that the duration of the second stage depends on the timing of the voluntary push. The duration was 12% shorter when the push was timed at B than when it was timed at A or C. This was the result of the higher peak total push force, the higher peak tissue strain, and the higher residual strain left after the strain relaxation of the viscoelastic tissues.FIGURE 3: Bar graph showing the effect of the voluntary timing on time to delivery (mins).CONCLUSIONS: In this computer simulation, the time to delivery was affected by the timing of the voluntary push relative to the peak uterine contraction.

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