What is the primary source of progesterone in an eight-month pregnant mare?

In an eight-month pregnant mare, the primary source of progesterone is the placenta. As the pregnancy progresses, particularly after the first few months, the placenta begins to take over the production of hormones that are critical for maintaining the pregnancy. During the early stages, the corpus luteum, which forms from the ovarian follicle after ovulation, produces progesterone to support the early pregnancy. However, around the middle of gestation, the placenta becomes the dominant source of progesterone, crucial for regulating and supporting the necessary changes in the mare’s reproductive system and maintaining the pregnancy until parturition.

While the corpus luteum continues to have a role in early gestation, its influence diminishes as the placenta assumes this critical hormonal function. The ovaries and adrenal glands are not significant sources of progesterone at this advanced stage of pregnancy in mares. The ovaries may still produce some hormones, but the emphasis shifts predominantly to the placenta for progesterone synthesis as gestation progresses.