Progesterone and estrogen concentrations in gilts with delayed parturition

Twelve pregnant gilts were assigned to three treatments (four gilts per treatment): (1) corn oil (control) injected subcutaneously four times daily from day 110 of gestation until onset of lactation; (2) 25 mg progesterone (P4) injected subcutaneously four times daily from day 110 of gestation through day 113, or (3) 25 mg P4 injected subcutaneously from day 110 through day 115 of gestation. Excluding one gilt on treatment 2 which farrowed on day 110, average lengths of gestation for control gilts and gilts receiving treatments 2 and 3 were 113.3 +/- 1.2 (SE), 115.6 +/- .7 and 118.0 +/- .4 days. Associated mean percentages of live births were 87.4 +/- 5.9, 48.8 +/- 24.4 and 2.5 +/- 2.5, respectively. P4 averaged 2.9 +/- .6 ng/ml in blood serum of control gilts during the 24 hr preceding parturition. P4 was released in a sporadic pattern by gilts on treatment 3, resulting in higher P4 concentrations (9.0 +/- 1.7 ng/ml) for the same prepartum period. Concentrations of P4 in serum of gilts on treatment 2 averaged 7.6 +/- 1.6 ng/ml, intermediate to, but not different from, concentrations of P4 in controls and gilts on treatment 3. Concentrations of estrogen in serum of control gilts averaged 6.2 +/- 1.3 ng/ml during the 24 hr preceding parturition, whereas concentrations of estrogen in gilts treated with P4 were reduced (P less than .01) to below 2 ng/ml during the same prepartum period. The erratic pattern of concentration of P4 during and after administration of exogenous P4 may reflect a disturbance of the secretion or metabolism of endogenous P4. We suggest that failure to suppress P4 post-treatment plus reduced estrogenic stimulation impedes the birth process and contributes to the high incidence of stillbirths observed in gilts with progestin-delayed parturition.