Hormone Supplementation during Pregnancy

During Pregnancy

During the first 120 days of gestation, progesterone is produced by the primary corpus luteum (CL). After 40 days of gestation, progesterone production by the primary CL is supplemented with progesterone produced by secondary CLs. Luteal production of progesterone persists through approximately Day 100 to Day 210 of gestation. Beginning around Day 50 to 70 of gestation, there is a measurable production of progestogen by the placenta, and the placenta is the only source of progestogen during the second half of gestation in the mare. Placental progesterone is rapidly metabolized to 5- pregnanes in the placenta. These 5-metabolites cannot be measured by most conventional progesterone assays. Therefore, after mid-gestation in the mare, the circulating progesterone concentration is low and does not accurately reflect progesterone production by the fetoplacental unit. In broad terms, one can consider that before 100 days of gestation the progesterone concentrations measured by the available assays reflect progesterone production by the maternal CL(s). After 150 days of gestation, progesterone concentrations represent only 1 to 5% of the total progestogens and are an unreliable parameter for assessment of placental progesterone production. Practically, at 100 days of gestation one can have a normal pregnant mare with essentially no measurable progesterone concentrations.

Products for progesterone supplementation

Like all steroid hormones, progesterone must bind to a specific progesterone receptor. The progesterone receptor across species has a specificity for progesterone (therefore it does not bind with estrogen or testosterone) however within each species this receptor has characteristics that allow it to bind with some synthetic progestagens and not with others. In the horse, altrenogest (aka allyltrenbolone or ReguMate®) is the only progestagen with proven bioactivity. Altrenogest is typically used at a daily dosage of 22 mg per os and progesterone-in-oil is used at a daily dosage between 150 and 300 mg administered intramuscular. Lower dosages or less frequent administration do not maintain blood levels does not maintain adequately levels between administrations.

Timing and Duration of Progestin Supplementation

In normal mares, progesterone supplementation can be started immediately after ovulation without adverse effects on conception rate and early embryonic development. However, in mares with post-insemination [...]