Plasma progesterone concentrations in pregnant and non-pregnant black bears (Ursus americanus).

Plasma levels of progesterone were determined from serial samples taken from 2 black bears over 3 consecutive fall periods. Each animal was pregnant during the 1st and 3rd years. Variations in progesterone levels were seen between animals and within each animal between pregnancies. Average baseline levels during the mid-to-late preimplantation period were 5.0-12.5 ng/ml and increased 2-3 fold at the approximate time of implantation. Values during non-pregnancy were detectable but much lower (0.6-2.5 ng/ml). The observed pattern of progesterone secretion in this species appears consistent with that reported for other species exhibiting obligate delayed implantation.