Changes in cervical resistance and collagen fluorescence during gestation in rats.

During pregnancy the role of the cervix shifts between two opposing functions. Throughout most of gestation, the cervix is rigid and resists tension in order to maintain the products of conception inside the uterus. At term, however, cervical function changes drastically in order to accommodate stretch and delivery. The events that control cervical function are not known. The aim of this study was to characterize changes in cervical resistance and collagen content during pregnancy in the rat. To determine the change in cervical resistance, non-pregnant and timed pregnant Sprague Dawley rats were sacrificed at various times. Their cervices were isolated and suspended in organ baths connected to a cervimeter for measurement of the stretch-tension relationship. In a different group of animals, cervical collagen content was measured using light-induced fluorescence in non-pregnant and, longitudinally, in pregnant rats. Cervical resistance and collagen content decreased progressively during pregnancy. The changes in cervical resistance mirrored those in cervical collagen content and the nadir in both occurred about two days prior to the onset of labor. Our study suggests that cervical preparation for delivery does not occur acutely at the time of labor and that cervical collagen content determines cervical resistance.