During pregnancy, after the first trimester, which structure is the major producer of progesterone?

• A. Placenta
• B. Corpus luteum
• C. Adrenal cortex
• D. Pituitary gland

Answer: (A)

The concept being tested is how progesterone production shifts during pregnancy. Progesterone is essential for maintaining the uterine lining and keeping the uterus relaxed to support the developing fetus.

Early in pregnancy, the corpus luteum in the ovary produces most of the progesterone, kept alive by human chorionic gonadotropin from the embryo. As the placenta develops, it takes over and becomes the primary source of progesterone. By the end of the first trimester, placental progesterone production is sufficient to sustain pregnancy, and the corpus luteum regresses as its support wanes. The placenta makes progesterone from cholesterol using its steroidogenic enzymes, ensuring a continued supply through the remainder of gestation. The adrenal cortex and pituitary gland do not provide the major progesterone output during this period, so the placenta is the main producer after the first trimester.