Quantifying Lower Birth Canal Resistance to Dilation During the 1st Stage of Labor

Use a novel device to quantify inter-individual differences in birth canal resistance to dilation during the late 1st stage of labor. The PREP device, a pelvic floor dilator, was introduced into the first 4 cm of the vaginal canal in nulliparous women during the first stage of labor (at approx. 6cm) during the EASE trial (NCT 03973281). A semi-automatic, dilation controlled actuation system mechanically expanded the device to ∼8 cm in a step-wise manner over ∼60 minutes. Information regarding the force with which the lower birth canal resists dilation and diameter was recorded continuously. The intrinsic mechanical properties of the birth canal during labor were obtained from force and dilation recordings. The viscoelastic behavior of the birth canal from the PREP device data were compared between individuals. Force and dilation measurements were available from 20 nullipara (mean age: 30.8 ± 4.2 years) in the late 1st stage of labor with adequate regional anesthesia (Fig. 1, two examples). The initial force with which the birth canal resisted initial dilation from 4 to 5.5 cm varied between 3 and 15 N (0.7 - 3.4 lbf), a ∼4-fold variation. During the 4-min hold at 5.5 cm that force decreased exponentially to between 3 and 8 N, a ∼3-fold variation reflecting differences in the stress-relaxation characteristics of the birth canal. During the entire 60-minute dilation to 8 cm, the maximum dilation force ranged from 10 to 28 N, a ∼3-fold variation. These differences are summarized graphically in Fig. 2 (top), along with the time points when they occurred Fig. 2 (bottom). These first-in-human measurements of lower birth canal resistance to dilation show large inter-individual differences in viscoelastic behavior. Future follow up studies are needed to evaluate the possible association between greater resistance to dilation and risk for levator ani injury.View Large Image Figure ViewerDownload Hi-res image Download (PPT)