Effect of relaxin on parturition and frequency of live births in pigs.

Gilts on Day 105 of gestation were sham ovariectomized (Group C, N = 5); ovariectomized and given i.m. injections of 100 mg progesterone twice daily from Day 105 to 112 (Group OP, N = 5); or ovariectomized and given progesterone and i.m. injections of 1 mg highly purified porcine relaxin 4 times/day from Day 105 until the end of parturition (Group OPR, N = 5). Concentrations of progesterone in peripheral plasma of gilts in Group OP were similar to those in Group C. Relaxin was undetectable in peripheral plasma of gilts in Group OP which also showed prolonged parturition (P less than 0.001) and impaired frequency of live births (P less than 0.001), although onset of lactation was not affected. In Group OPR the duration of parturition and frequency of live births were similar to those observed for gilts in Group C. These results indicate that the ovarian hormone relaxin is necessary for normal duration of parturition and frequency of live births and that the onset of lactation is not prevented by an absence of relaxin.