PROGESTERONE IN BLOOD

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Abstract
SUMMARY: Progesterone levels have been measured in the peripheral blood of pregnant and non-pregnant mares, and in umbilical cord blood collected during foaling. It was found that progesterone was readily detectable in the blood of pregnant and non-pregnant animals provided that there was a fully developed corpus luteum present in the ovaries. During the second half of gestation, when the ovaries have become fibrotic, progesterone was no longer detectable in the peripheral blood, even though it was present in high concentrations in umbilical cord blood. The reasons for this apparent disappearance of progesterone from the maternal circulation are discussed, and it is concluded that during the second half of gestation placental progesterone probably exerts a local action on the uterus, without even entering the maternal circulation. This local action is made possible because of the diffuse, epithelio-chorial nature of the mare's placenta.