Effects of Fetal Head Motion on Pelvic Floor Mechanics

Abstract

We have developed a modelling framework to simulate vaginal delivery using individual-specific pelvic floor geometries derived from magnetic resonance images, which could potentially be used to study the effect of various contributing factors (such as muscle mass and muscle tone) on the childbirth process. One important goal of this is to faithfully represent the path of the fetal head as it descends through the maternal pelvis. Previous studies have prescribed or restricted the path of the fetal head. In this study, we investigate the effect of the fetal head motion on the mechanical response of the pelvic floor muscle by applying three types of constraints to the fetal head: (1) a completely prescribed head path; (2) allowing rotation about one point on the head; and (3) a minimally constrained head, allowing both rotation and translation. The results were analysed by comparing the normalised reaction forces exerted on the fetal head and the maximum principal stretch ratios of the pelvic floor muscles. The reaction forces for case (3) were markedly reduced compared to cases (1) and (2), indicating that the path of the fetal head has significant influence on the force required to achieve delivery. These results also indicate that models should use a minimally constrained fetal head so that its path is determined by the geometry of the head and the pelvic floor. Future work will introduce nonlinearity and anisotropy into the pelvic floor muscles and use this modelling framework to compare the mechanical response between athlete and non-athlete in order to determine the factors that contribute to difficulties during the second stage of labour for athletic mothers.