Serum Relaxin Concentrations in Pigs with Parturition Delayed by Progesterone Administration*

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Abstract
Two groups of pregnant pigs were administered four sc injections of 25 mg progesterone each day from day 110 through day 113 (P4) or from day 110 through day 115 (P6) of gestation. In control pigs, which received only corn oil, the mean gestation length was 113.8 days and the mean incidence of live births was 87%. P4 had a mean gestation length of 115.7 days and a mean incidence of live births of 49%. These values did not differ significantly from controls. P6, however, had a longer mean gestation length (P < 0.01) of 118.1 days and a lower mean incidence of live births (P < 0.001) of 3% than controls. Progesterone treatment did not influence the time of occurrence of the relaxin surge which normally takes place during the 2 days immediately preceding parturition. The mean days of gestation on which maximal relaxin concentrations were obtained for controls, P4, and P6 were 113.0, 112.9, and 112.6, respectively. The mean time interval between maximal relaxin levels and parturition was 22.5 h for controls. Longer time intervals of 70.0 h (P < 0.05) and 136.5 h (P < 0.001) were obtained with P4 and P6, respectively. Progesterone treatment also did not influence the onset of lactation, which was recorded as the time when milk could be readily expressed from the teats. The mean days of gestation on which the onset of lactation was observed for controls, P4 and P6 were 113.8, 114.4, and 113.1, respectively. In all cases the onset of lactation occurred within 24 h following the surge in serum relaxin levels. (Endocrinology102: 471, 1978)