Progesterone is the key hormone responsible for maintaining pregnancy in all mammals, including horses. A blood progesterone concentration of ≥ 4 ng/ml is considered adequate to maintain the pregnancy during the first 2-3 months of gestation. At this time period in the mare, the corpus luteum – a gland that develops on the ovary after ovulation – is the primary source of progesterone. It is less common and less clinically relevant to measure blood progesterone levels after 100 days of pregnancy. Collection of a blood sample for measurement of progesterone concentration can determine if a pregnant mare is producing sufficient progesterone to maintain her pregnancy.

There are several clinical indications for measurement of blood progesterone levels in a pregnant mare, including:

  • Ultrasound examination and detection of a small corpus luteum in an early pregnant mare; a small corpus luteum may be associated with a low progesterone level
  • Ultrasound examination and detection of endometrial edema during early pregnancy; the presence of edema suggests that progesterone levels are low
  • Monitoring progesterone levels in a problem mare or a mare with repeated pregnancy loss
  • Monitoring progesterone level in a pregnant mare being supplemented with altrenogest (Regu-Mate® or similar generic product) to determine if progestin therapy can be discontinued
    • Altrenogest is not detected in most progesterone assays
    • A blood progesterone concentration ≥ 4.0 ng/ml is theoretically sufficient to maintain pregnancy without additional supplementation

Should a mare’s progesterone level on the analysis be < 4.0 ng/mL, progesterone supplementation is indicated to help maintain the pregnancy. Options for progesterone supplementation at the Equine Reproduction Laboratory include:

  • Altrenogest (Regu-Mate®, a synthetic progestin) administered orally once daily
  • Natural progesterone administered by intramuscular injection.
  • Short-acting progesterone is administered once daily
  • Long-acting progesterone is administered once per week

The choice of progesterone supplementation is based on the clinical situation, the ability of the mare to tolerate oral or intramuscular medications, owner preferences, and other factors.