Plasma progesterone concentrations during the estrous cycle, pregnancy, and lactation in the Chinese hamster,...

From vaginal smear and histological observations, Chinese hamster (Cricetulus griseus) exhibited four day estrous cycles and had a gestation period of 20 days. The concentrations of progesterone fluctuated in a biphasic pattern during the estrous cycle with peak values at diestrus and proestrus. The levels of the hormone increased approximately threefold from day five to day eight of gestation, remained elevated until parturition, and then fell precipitously during lactation to levels on day zero of gestation.