Plasma levels of progesterone, cortisol, and corticosterone in the pregnant rabbit.

Abstract

Peripheral plasma levels of progesterone, cortisol, and corticosterone were measured every other day throughout pregnancy in primiparous rabbits by competitive protein-binding techniques. Progesterone concentrations rose immediately after breeding and reached peak levels on Days 16–18, after which there was a decrease to prepartum levels by Day 32. Corticosterone remained constant for the first 24 days of gestation (7.3 ng/ml) with a slight rise to 17.0 ng/ml during the last few days of pregnancy. Cortisol levels (7.1 ng/ml) also remained constant for the first 24 days of gestation, although, in contrast to corticosterone, an abrupt increase began on Day 26 with peak levels (96.7 ng/ml) on Day 30. By Day 32, cortisol levels were similar to those in early pregnancy. Declining progesterone levels plus increased cortisol levels appear to be important preparturient changes in the rabbit.