The levels of progesterone in the ovarian venous blood were found to be high during the second half of gestation, ranging from 958 ± 210 ng/ml to 3096 ± 484 ng/ml. The temporal pattern of progesterone concentration in the ovarian venous plasma showed a biphasic variation during the second half of gestation, with maximum values on Day 13 (3096 ± 484 ng/ml) and Day 16 (2778 ± 265 ng/ml).

Uterine venous plasma progesterone concentrations were measured in pregnant mice which were unilaterally ovariectomized 24 h prior to bleeding. Progesterone concentrations were also measured in the systemic plasma from the same animals. Plasma progesterone concentrations in the uterine venous blood were higher than those observed in the systemic circulation of the same animals, especially on Day 16 and Day 17 of gestation.

While these results confirm that the ovary is the primary source of progesterone synthesis during pregnancy in the mouse, the higher levels of progesterone in the uterine venous blood compared with levels in the peripheral blood, strongly suggest that the mouse placenta synthesizes small amounts of progesterone during the second half of gestation. At least a part of this progesterone is secreted into the maternal circulation.