The time of increase in plasma progesterone during pregnancy in mink (Mustela vison ).

In 1980 and 1981, respectively, 12 and 19 one-year-old mink females of standard and jetblack breeds were used to determine the progesterone level approximately at the time of expected implantation. Blood samples were collected every 2 to 3 days or daily in order to accurately estimate the time of increase in plasma progesterone levels. The results indicate that the progesterone level increased to above 10 ng/ml, 31.6 (sigma = 1.3) days prior to parturition on the average. This was supported by physiological explanations. The date of mating in mink had an effect on the date of increase in plasma progesterone. Matings at a late date in the estrous period, reduced the period of delay before implantation. Nevertheless, both the dates of implantation and parturition were delayed compared with the results of earlier matings in mink.