Which structure secretes progesterone before placental circulation and is seen as a thin-to-thick hyperechoic wall with hypervascularity, usually about 5 cm in diameter?

• A. Corpus luteum
• B. Ovarian cyst
• C. Theca lutein cyst
• D. Follicle

Answer: (A)

During early pregnancy the corpus luteum forms after ovulation and is responsible for secreting progesterone until the placenta takes over that role. On ultrasound it appears as a cystic structure within the ovary with a relatively thick, sometimes echogenic wall. Color Doppler often shows increased blood flow around the wall—the peripheral or ring-of-fire pattern—reflecting its hormonally active luteal tissue. Its typical size is around 5 cm in diameter. This makes it the structure described, since it is the source of progesterone before placental circulation begins. A simple ovarian cyst would usually have a thin wall and lack this characteristic peripheral vascularity; a follicle before ovulation is typically smaller with a thinner wall and different Doppler appearance; and theca lutein cysts are generally multiple and associated with high hCG states rather than a single, progesterone-secreting corpus luteum.