A corpus luteum cyst is a normal, harmless, fluid-filled mass that forms in your ovaries. It grows on your corpus luteum, a group of cells that take shape after your ovary releases an egg (ovulation) each month. It has an important function during pregnancy.
Your corpus luteum produces a hormone called progesterone that your uterus needs to house a developing fetus. It breaks down when it’s no longer needed. A cyst forms when your corpus luteum keeps growing.