Concentration increase of unbound testosterone in plasma of the mare throughout pregnancy.

Blood testosterone levels were measured by RIA and gas chromatography-mass spectrometry in the pregnant mare. They were found to increase from the very beginning of pregnancy, reaching peak values 10 times higher than the basal values at the seventh month and then to return to basal values by the week after parturition. Testosterone binding by plasma proteins was investigated in nonpregnant and pregnant mares throughout gestation. Equilibrium dialysis and gel equilibration methods did not reveal any blood specific testosterone-binding activity at any gestational stage. Hence, blood testosterone increase cannot result as in the pregnant woman or guinea pig, from an increase in blood specific binding activity but more probably results from an increased steroid production. It is suggested that the maternal gonads would be responsible for the testosterone increase in early pregnancy, during the period of PMSG production, and the fetoplacental unit for the subsequent increase.