Effects of relaxin administration in early gestation or prior to mating on uterine length and fetal survival in gilts.

The objective of this research was to determine the effect of administration of porcine relaxin to gilts during early gestation, or during the follicular phase of the estrous cycle immediately preceding mating, on the length of the uterus and consequently the number of surviving fetuses. Experiment 1 determined the individual and combined effects of 10 days of administration of relaxin (0.5 mg, 4 times daily), estradiol benzoate (1 mg, 2 times daily), and progesterone (50 mg, 2 times daily) on uterine wet weight and length in 58 ovariectomized 8-mo-old pubertal gilts. Relaxin alone had no effect on either uterine length or wet weight. Estrogen increased uterine wet weight, and this effect was augmented by relaxin. Progesterone increased uterine length, and this effect was augmented by estrogen. Combined treatment with progesterone, estrogen, and relaxin increased both uterine length and wet weight maximally. Experiment 2 determined the effects of relaxin administration in early gestation or prior to mating on uterine length and fetal survival in 75 unilaterally ovariectomized-hysterectomized gilts. Relaxin (0.5 mg, 4 times daily) was administered during three treatment' periods, and uteri were collected on Day 40 of gestation. In this study, relaxin administration from Days 11 to 20 of gestation reduced the number of live fetuses (p=0.01) and percentage survival (p=0.01) and resulted in shorter uterine length (p=0.01) and lower uterine wet weight (p=0.03) than in controls, but did not affect length of uterus per fetus or uterine dry weigt. Relaxin administration from Days 22 to 31 of gestation did not influence fetal survival, uterine length, uterine length per fetus, uterine wet weight, or uterine dry weight. Relaxin administration from Days 11 to 20 of the estrous cycle followed by mating at the subsequent estrus reduced the number of live fetuses (p=0.03), reduced the percentage survival (p=0.03), increased the occupied length of uterus per live pig (p=0.02) relative to controls, and did not affect uterine length, uterine wet weight, or uterine dry weight. It is concluded that, when administered during early pregnancy or during the estrous cycle preceding mating as described in this study, relaxin increases neither uterine length nor fetal survival in pregnant gilts