Biomechanical adaptations of the rat vagina and supportive tissues in pregnancy to accommodate delivery

We hypothesize that in pregnancy and at the time of delivery, the vagina and supportive tissues undergo dramatic alterations to accommodate passage of the fetus. In this study, we sought to characterize these changes in the rat using an established biomechanical testing protocol.

Seventy-four 3-month-old Long Evans rats divided into virgin, mid and late pregnant, vaginal delivery (immediate and 4-week postpartum), and abdominal delivery (immediate and 4-week postpartum) groups were killed. The biomechanical properties of the vagina and supportive tissues were tested intact as a complex under loading conditions that simulate downward distension. Data were analyzed using analysis of variance and post hoc comparisons.

Mean linear stiffness (ability of the specimen to resist distension) and ultimate load at failure (maximal resistance of the specimen to distension before disruption) were decreased in pregnancy and at delivery, regardless of delivery route (Ps<.001). Maximal distension was increased at time of delivery (Ps<.001). Four weeks after vaginal delivery, all biomechanical characteristics returned to at least virgin values.

In the rat, the biomechanical characteristics affording distensibility of the vagina and supportive tissues increased in pregnancy and even further at delivery. It is likely that these represent maternal tissue adaptations that facilitate delivery of the fetus(es).