The corpus luteum is a temporary endocrine structure that forms from the collapsed dominant follicle after ovulation. On ultrasound it typically appears as a thick-walled cystic structure with internal echoes or clot, often with a characteristic ring of fire pattern on color Doppler due to its rich peripheral vascularity. It produces progesterone to support early pregnancy. A corpus luteum cyst can persist and grow up to 3–5 cm, occasionally causing pain or a pelvic mass that must be distinguished from an ectopic pregnancy or ovarian neoplasm.