When a follicle releases its egg, it begins
producing estrogen and progesterone for conception. This follicle is now called the corpus luteum. Sometimes, fluid accumulates inside the follicle, causing the corpus luteum to grow into a cyst.
Not related to the menstrual cycle:
Dermoid cysts. Also called teratomas, these can contain tissue, such as hair, skin or teeth, because they form from embryonic cells.
Cystadenomas.