Changes in plasma progesterone levels during the estrous cycle and pregnancy in 4-day cyclic mice

Changes in plasma progesterone levels during the estrous cycle and pregnancy were determined in group-housed IVCS strain mice showing regular 4-day cycles in the absence of a male. The pattern of progesterone secretion during the estrous cycle consisted of a large peak and a small one, the former during the late afternoon of proestrus and the latter on the afternoon of metestrus. From Day 0 to Day 2 of pregnancy, a sharp rise was observed, and then this level reached a plateau. A marked increase in progesterone was shown from Day 13 until the day before parturition.