OBJECTIVE The purpose of this study was to identify pregnancy-induced changes in biomechanical properties of the vaginal wall and to compare these with fibulin-5 knockout mice (Fbln5(-/-)) with and without prolapse. STUDY DESIGN Mid-vaginal segments of nonpregnant and late-pregnant wild-type mice, Fbln5(-/-) with prolapse mice and Fbln5(-/-) mice without prolapse were studied. Tissue length at failure, maximal strain, maximal stress, and tissue stiffness were determined. RESULTS Compared with nonpregnant mice, vaginas of pregnant and Fbln5(-/-) (with prolapse) mice exhibited decreased maximal stress, increased distensibility and strain, and decreased stiffness. Tissues from Fbln5(-/-) mice without prolapse were similar to nonpregnant wild-type animals. CONCLUSION Pregnancy confers remarkable changes in the vaginal wall that include increased distensibility and decreased stiffness and maximal stress. Elastinopathy alone is insufficient to cause significant changes in these properties, but prolapse confers additional alterations in distensibility and stiffness that are similar to those changes that have been observed in pregnancy. These changes may contribute to the poor durability of many restorative surgical procedures for prolapse.