Comparison of Serum Progesterone, 20α-Dihydroprogesterone, and Estradiol-17β in Pregnant and Pseudopregnant Rabbits: Evidence for Postimplantation Recognition of Pregnancy1

Progesterone, 20α-dihydroprogesterone, and estradiol-17β were measured in serum of pregnant and pseudopregnant New Zealand White rabbits. Neither progesterone concentrations nor progesterone gradients on consecutive days differed between these two reproductive states until several days after implantation. No differences in concentrations of 20α-dihydroprogesterone were found between pregnant and pseudopregnant rabbits during the 28 days following mating. Estradiol-17β profiles were similar in pregnant and pseudopregnant rabbits during Days 1–6 and Days 12–17 following mating, with the exception of an apparent rise in estradiol-17β concentration in pseudopregnant does during luteal regression. These data support the concept that an alteration in ovarian function in response to the presence of embryos does not occur until several days after implantation in the rabbit. Furthermore, the available data clearly indicate that regression of the corpus luteum at the end of pseudopregnancy is not caused by withdrawal of estradiol, the luteotropic hormone. Rather, the corpus luteum regresses in the presence of increasing estradiol concentrations accompanying the return to estrus.