Pelvic organ prolapse and urinary incontinence are common conditions in women that significantly diminish quality of life. Vaginal delivery and maternal birth injury are the number one risk factors for the development of pelvic floor disorders. The goal of this study was to characterize maternal adaptations throughout pregnancy and recovery after vaginal delivery in terms of the passive quasi-static mechanical properties of the vagina using a rodent model.
Virgin (n = 8), mid-pregnant (n = 7, day 15–16), late-pregnant (n = 7, day 20–21), immediate postpartum (n = 8, <2 hours post delivery), and 4 week postpartum (n = 6) Long-Evans female rats were utilized in this study. The mechanical properties (tangent modulus, tensile strength, ultimate strain, and strain energy density) were quantified by testing longitudinal sections of vaginal tissue to failure.
The tangent modulus of virgin animals (25.1±5.1 MPa) was significantly higher compared to mid-pregnant (11.7±7.7 MPa, p = 0.003), late-pregnant (7.9±4.0 MPa, p<0.001), and immediate postpartum (8.5±4.7 MPa, p = 0.001) animals. A similar trend was also observed in the tensile strength, whereas the ultimate strain increased throughout pregnancy until the time of vaginal delivery. Recovery was observed four weeks postpartum as no significant difference was found from virgin animals for any of the parameters.
This study has shown a significant decrease in the tangent modulus and tensile strength along with an increase in the ultimate strain of longitudinal sections of vaginal tissue throughout pregnancy. These maternal adaptations are likely to increase the overall distensibility of the vagina and allow for vagina delivery with minimal injury. This process appears to be effective in the rodent model as the properties recovered to virgin levels by 4 weeks. In the future, we hope to alter these adaptations or exceed them in order to study the risk and impact of birth injury in this model.