Induction of labor with RU 486 (mifepristone) in relaxin-deficient rats: antepartum administration of relaxin facilitates delivery and increases pup survival

Objective This study was conducted to determine whether antepartum administration of relaxin improves RU 486–induced delivery at term in rats that lack circulating endogenous relaxin. Study design Pregnant rats were modified two ways to obtain circulating levels of relaxin and progesterone that resemble those of pregnant humans: relaxin was immunoneutralized throughout the second half of the 23-day pregnancy and high progesterone levels were sustained until term by inserting progesterone implants on day 20. Porcine relaxin was administered subcutaneously from 8 on day 20 until delivery. Labor was induced by administering RU 486 subcutaneously at 4 on day 22. Results After induction of labor with RU 486, labor and delivery were faster, and the incidence of live births was higher when rats were also administered relaxin during the antepartum period. Conclusion Antepartum administration of relaxin in combination with RU 486 has beneficial effects on delivery in relaxin-deficient rats.