Effect of pregnancy, vaginal delivery, postpartum remodeling, and age on the tensile behavior and biochemical composition of the murine ulterosacral ligament (USL)

Pelvic organ prolapse (POP) is a debilitating condition affecting millions of women. The uterosacral ligament (USL) provides primary apical support for the uterus and pelvic organs. Advanced POP is associated with extracellular matrix degradation and decreased USL mechanical integrity. Pregnancy, vaginal delivery, multiparity, and age are inciting risk factors for POP, but it is unknown how these individual factors affect USL structure and function. In 10-to-15 weeks old, young adult mice, we investigated how USL mechanical behavior and biochemical composition were altered from a non-pregnant (nulliparous) baseline by pregnancy, delivery, and postpartum recovery at 4 weeks. The effect of multiparity and age on USLs was also evaluated in ∼8-month-old multiparous and nulliparous mice. Stress relaxation testing was used to evaluate USL mechanical behavior, and Raman spectroscopy assessed tissue biochemistry. Concurrent histoarchitectural changes were evaluated in the vagina, bladder, and rectum. Pregnancy and delivery significantly reduced the apparent elastic modulus and increased viscous relaxation in young adult USLs. USL mechanical behavior recovered to nulliparous levels after pregnancy by 4 weeks postpartum and in multiparous mice. Collagen remodeling occurred in USLs following pregnancy, delivery, postpartum recovery, and age. Thus, USLs soften during pregnancy and delivery, likely in preparation for the rigors of birth. Following delivery, mechanical recovery occurred despite persistent biochemical changes with postpartum recovery and aging. Overall, subtle but measurable changes in USL mechanical behavior and composition were associated with risk factors of pregnancy, delivery, and age, indicating that these inciting factors may cumulatively contribute to the development of POP. Statement of significance: Pelvic organ prolapse (POP) is a common multifactorial condition, yet its etiology remains unclear. Advanced POP occurs with failure of uterosacral ligaments (USLs) which provide the apical support of the pelvic organs. This is the first study to examine how risk factors for POP, including pregnancy, vaginal delivery, multiparity and age, alter USL mechanical behavior and extracellular matrix composition. USL mechanical properties were transiently altered during pregnancy and delivery but recovered, whereas tissue compositional changes occurred with multiparity and independently with aging. The surrounding pelvic-region tissues also experienced compositional and organizational changes. Combined, these biomechanical and compositional changes may shed light on the multifactorial nature of POP, providing insight into the specific cause-and-effect mechanisms by which these risk factors compromise pelvic support tissues.