In an eight-month pregnant mare, what is the source of progesterone?

• A. Placenta
• B. Corpus luteum
• C. Adrenal gland
• D. Ovarian tissue

Answer: (A)

Late in gestation, the placenta begins to take over progesterone production to maintain the pregnancy. In the mare, while the corpus luteum supports early and much of the pregnancy, the placental tissue increases its activity as gestation progresses and becomes the primary source of progesterone by late pregnancy. This progesterone helps keep the uterus quiet and maintains the endometrium in a state suitable for the developing fetus, reducing the risk of premature uterine contractions as parturition approaches. The adrenal gland and ovarian tissue contribute far less prolonged progesterone, so they are not the main sources in an eight-month pregnancy.