Estradiol and Progesterone Profiles Indicate a Lack of Endocrine Recognition of Pregnancy in the Opossum

Concentrations of estradiol and progesterone in blood collected during the 12.5-day gestation period of the Virginia opossum were not significantly different from those during equivalent days of the estrous cycle. Progesterone was correlated with an index of corpora luteral mass. Ratio of estradiol to progesterone were highest 3 to 4 days before estrus and on the day of parturition.