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Plasma progesterone levels during pregnancy and pseudopregnancy in the hare (Lepus europaeus syriacus)

Summary. A triphasic pattern of progesterone secretion was observed in female hares sampled throughout pregnancy and pseudopregnancy. After injection of hCG and artificial insemination (Day 1), progesterone values rose to a peak of 41 ·4 ng/ml about Day 14, remained at this level, then declined around Day 20 before increasing sharply to maximum levels of 67·7 ng/ml after midpregnancy (Day 28). Levels remained high for several days, then declined until Day 38, increased again until Day 41, before decreasing towards parturition. Progesterone levels were still high (37·5 ng/ml) 24 h before parturition. The progesterone pattern during pseudopregnancy closely resembled that observed during the first half of pregnancy: levels rose from Day 2 to a peak at Days 11–18, then declined sharply to baseline levels around Day 22. It is suggested that the control of progesterone secretion might be transferred from the pituitary to the placenta at the beginning of the second half of pregnancy.