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

Abstract

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 which characterizes how long it takes the birth canal to be dilated by the fetal head. That hypothesis was rejected because τ 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 values, with a noticeable increase for τ values over 1000 s. The correlation coefficent between predicted and observed second stage durations was 0.51. We conclude that τ 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.

Contraction of the levator ani muscle during Valsalva maneuver (coactivation) is associated with a longer active second stage of labor in nulliparous women undergoing induction of labor

“…Viscoelastic properties of the distal birth canal have been suggested as a strong contributor to the time a mother needs to push in the second stage to deliver the fetal head. 31 Previous studies used transperineal ultrasound to investigate the relation between antenatally assessed pelvic organ mobility on Valsalva and levator ani hiatal dimensions in the prediction of outcome of labor and reported that reduced mobility and smaller levator ani hiatal dimensions are associated with increased risk of operative delivery. 33e35 However, other authors did not find any association between pelvic floor dimensions and the mode of delivery.…”

“…24,26,28e30 Indeed, it has been suggested that the viscoelastic properties of the intact distal birth canal in healthy nulliparous women may predict the duration of the second stage of labor. 31 However, the effect of levator ani muscle coactivation on labor outcome in women undergoing induction of labor has not been previously assessed.…”

Letter to the editor of heliyon re: The predicting value of the ratio of levator hiatus diameter to fetal head circumference in pregnant women at 37 weeks of gestation in the progression of the second stage of labor and levator ani injury 6 weeks postpartum”

“…They directly tested that idea using a computer model based on the published variations in the two parameters constituting the Capacity-Demand ratio. They used a similar approach for the risk for risk of levator injury which they refined in a follow-up paper, Tracy et al [ 4 ] by adding actual measures of distal maternal birth canal tissue elasticity and viscosity during the first stage of labor to predict the length of the active second stage of labor and risk for levator injury, both goals of the Gan et al paper.…”