Changes in the tensile properties and fine structure of the rat cervix in late pregnancy and during parturition.

The isolated cervix from non-pregnant, early Day 22 pregnant (late pregnant) and parturient rats was studied. Cyclic loading and unloading tensile tests showed that the creep properties of the cervix were greater in late pregnant than in non-pregnant rats. This change in tensile properties, or softening, was associated with a marked rise in % of water and a fall of collagen concentration (as % of wet but not dry weight). These findings, plus electron micrographs showing a marked increase in extracellular matrix, separation of bundles of collagen fibrils and active fibroblasts, suggest that softening if related to controlled tissue hydration. Cervices were also removed from 3 groups of rats killed at different times during the 3-h parturient period and creep rate measured by continuous loading. A nearly 3-fold increase in creep rate over this period indicates that there is a second stage of cervical softening in pregnancy which precedes dilatation at parturition.