On the variation in maternal birth canal in vivo viscoelastic properties and their effect on the predicted length of active second stage and levator ani tears

The pubovisceral muscles (PVM) help form the distal maternal birth canal. It is not known why 13% of vaginal deliveries end in PVM tears, so insights are needed to better prevent them because their sequelae can lead to pelvic organ prolapse later in life. In this paper we provide the first quantification of the variation in in vivo viscoelastic properties of the intact distal birth canal in healthy nulliparous women using Fung's Quasilinear Viscoelastic Theory and a secondary analysis of data from a clinical trial of constant force birth canal dilation to 8 cm diameter in the first stage of labor in 26 nullipara. We hypothesized that no significant inter-individual variation would be found in the long time constant, τ ₂ which characterizes how long it takes the birth canal to be dilated by the fetal head. That hypothesis was rejected because τ ₂ values ranged 20-fold above and below the median value. These data were input to a biomechanical model to calculate how such variations affect the predicted length of the active second stage of labor as well as PVM tear risk. The results show there was a 100-fold change in the predicted length of active second stage for the shortest and longest τ ₂ values, with a noticeable increase for τ ₂ values over 1000 s. The correlation coefficent between predicted and observed second stage durations was 0.51. We conclude that τ ₂ is a strong theoretical contributor to the time a mother has to push in order to deliver a fetal head larger than her birth canal, and a weak predictor of PVM tear risk.