Time-dependent deformation of the vagina during gestation under controlled pressure

The vagina undergoes extraordinary structural and biomechanical changes throughout pregnancy to facilitate delivery. To characterize the magnitude and temporal progression of these changes, this experimental study quantified the inflation-induced creep response of the murine vagina at multiple gestational time points. Vaginas from non-pregnant (NP), mid-pregnant (MP), and late-pregnant (LP) mice were subjected to two creep tests at luminal pressures of 7 and 14 kPa using a custom free-extension inflation apparatus. Resulting deformations in the longitudinal and circumferential directions were quantified using digital image correlation. Collagen microstructure in NP, MP, and LP vaginas was further examined using second harmonic generation imaging. The MP vagina was significantly stiffer than the NP and LP vagina, suggesting that the organ may stiffen through early-mid pregnancy and then regain compliance toward late pregnancy. Gestation-dependent differences in creep behavior were observed, with the LP vagina exhibiting greater creep than the MP vagina at 7 kPa and more than the NP vagina at 14 kPa. These mechanical adaptations were consistent with gestation-dependent changes in collagen microstructure, including transient fiber reorientation and increased dispersion in MP tissue, followed by partial recovery in LP tissue. The data shed new light on how the vagina remodels in preparation for delivery, and may contribute to future discoveries and developments for improved maternal care. Statement of Significance: This study characterizes the time-dependent mechanical behavior of the vagina during pregnancy in a murine model. Using inflation-based biaxial loading and digital image correlation, we quantify deformation during incremental pressurization to prescribed levels and over time at constant pressure. The vagina exhibits reduced compliance at mid-pregnancy and greater time-dependent deformation under sustained loading (creep) in late pregnancy. These changes are associated with collagen remodeling, including alterations in fiber organization, as assessed by second harmonic generation imaging. This behavior corresponds to a transition from a mechanically supportive state that may help maintain pregnancy to a more compliant response that accommodates parturition and reduces the risk of injury. These findings define how the vagina adapts during pregnancy and provide a basis for predictive models of tissue behavior that may help identify abnormal remodeling and improve maternal health outcomes.