Onapristone and prostaglandin E2 induction of delivery in the rat in late pregnancy: A model for the analysis of cervical softening

OBJECTIVES: Our purpose was to develop a method to induce premature delivery in rats and to use this method to identify biochemical changes that are critical to cervical dilatation by comparison to changes that occur during term delivery. STUDY DESIGN: Rats were treated with antiprogestational agents onapristone or lilopristone in combination with prostaglandin E2 or estradiol on day 19 of pregnancy to induce delivery before term. Mechanical and biochemical changes of the isolated cervix were compared with changes found at term and in 20-day controls. RESULTS: Rats treated with a combination of onapristone and prostaglandin E2 were consistently delivered 25 hours after treatment began. The physical characteristics of the cervix of treated rats changed to match those of term cervices. The ratio of the small sulfated proteoglycan (decorin) to collagen changed on induction to match the ratio found at term. CONCLUSIONS: This induction protocol can be used to advance the time of delivery in rats, with the further advantage that the time of delivery can be accurately predicted. The data strengthen a proposed model in which the interaction of decorin and collagen is an important determinant of the biomechanical properties of the cervix during pregnancy. (Am J Obstet Gynecol 1996;175:719-23.)