The effect of labor on the rheologic response of chorioamniotic membranes.

A study of chorioamniotic membranes was undertaken in 2 term populations. One group of women delivered vaginally after normal labor (N = 41), the other by elective cesarean section (N = 41). Rheologic properties were measured in an experimental environment. The membranes in each group responded similarly while intact. Those membranes subject to labor, however, ruptured earlier in cyclic testing. They also required more time to dissipate applied stress. This stress response pattern is similar to that seen with rupture of amorphous materials in which multiple flaws or cracks are aggravated by stress testing. The clinical problem of premature rupture of the membranes may occur by a phenomenon that produces flaws in their structure. The data suggest that these flaws may be aggravated by labor-like uterine activity.