Progestogens are a class of steroid hormones largely responsible for sustaining the embryo and maintaining uterine quiescence. In horses, at least 10 known progestogens are present in maternal circulation during gestation. To date, only a few of them are known to be biologically active.

Progesterone, the most renowned of this class of steroid hormones, is the only one with clinical diagnostic application. During early pregnancy, progesterone is produced in the equine ovary by the corpus luteum (CL), and its concentrations remain elevated and peak between 60 and 120 days of gestation. From that point on, progesterone slowly decreases until it becomes nearly undetectable around 180 to 200 days of gestation. During late gestation, other progestogens produced by the fetoplacental unit are responsible for maintaining the pregnancy. These are first detectable by Day 60 of gestation and are completely capable of maintaining pregnancy from around 120 to 140 days of gestation until term.

Circulating progesterone has been used diagnostically to evaluate luteal function during early pregnancy. When the circulating progesterone (P4) concentration is above 1 ng/mL, this is considered consistent with the presence of luteal tissue, indicating that a follicle has ovulated, luteinized and is producing progesterone. When the circulating progesterone concentration is above 4 ng/mL, this is considered adequate to maintain pregnancy. There are a number of reasons for monitoring and supplementing endogenous progesterone with progestins (synthetic progesterones) during pregnancy, such as uterine infections, history of pregnancy loss, and luteal insufficiency.

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