Abstract:
Up to 19% of nulliparous women sustain pelvic muscle injuries while giving vaginal birth. Since these injuries, which are never repaired, can have life-long sequelae including prolapse and incontinence, it seems reasonable to try to prevent them. From a biomechanical perspective, women with unusually stretch-resistant lower birth canals can expect longer labors and a higher risk for muscle and tissue injury. The goals of this dissertation were to (a) quantify the viscoelastic properties of the lower birth canal during the first stage of labor, with special attention to the effects of age and body mass index; (b) identify those at risk for a long second stage of labor, as well as (c) an elevated risk for levator ani injuries. Following an introductory chapter (Chapter 1), we describe the biomechanics of a new device that dilates the lower birth canal (PREP, Materna Medical, LLC., CA) during the first stage of labor, and its use in the ongoing multicenter EASE clinical trial (Chapter 2).

Next, we quantify the normal ranges of lower birth canal resistance to dilation (tension T) in 56 nulliparas during the first stage of labor. We found remarkable variation in birth canal wall tension across all subjects. Specifically, the tension when reaching 55 mm dilation varied 5.5-fold and was 22% greater in older women and correlated strongly with both relaxation during the 5-minute hold and with the maximum tension (r= 0.80, r = 0.59 with p<0.001), thus a reasonable predictor of resistance to lower canal dilation (Chapter 3). The stiffness of the initial 20 s ramp varied 6.8-fold and was 31% higher in older women.

We then demonstrated that the effect of the initial 20 s ramp-and-5 minute hold of PREP insertion on the viscoelastic properties of the lower birth canal can reliably be characterized using a fractional viscoelastic Zener model (FZM). In 61 nullipara, the relative error for the 20 s ramp-and-5 minute hold using the FZM was 8.1 ± 3.6% and provided a relative prediction error in 60 minutes of dilation of 10.0 ± 5.4%. The FZM constant kβ, an analog for the shorter timer constant, was 33% higher in older women (Chapter 4). We then used the fractional Zener viscoelastic constants obtained in Chapter 4 to predict the duration of the second stage of labor and risk for levator ani injury. As labor outcomes are presently unavailable for the PREP subjects due to the ongoing nature of the EASE trial, the model was tested on data from an earlier constant force PILOT dilator study involving 22 healthy women with available labor outcomes. Our prediction model identified both instances of LA injury and predicted 22% of the variation in the second stage of labor duration (Chapter 5). We conclude that the viscoelastic properties of the lower birth canal in the first stage of labor are responsible for nearly a quarter of the variation in second stage duration. When combined with existing obstetric variables, the PREP dilator may prove helpful in the improved management of vaginal delivery.

Zoom Link: https://umich.zoom.us/j/99452208073 Meeting ID: 994 5220 8073 Passcode: 0000

Committee Chair(s): Dr. James Ashton-Miller