How big do follicles get before ovulation?

Follicles grow to 3 to 4 millimeters before a single one is selected. When one is selected, it gains fluid and generally reaches 18 to 24 millimeters, similar in size to a small grape or olive.

Each follicle that grows in the ovaries during the menstrual cycle contains an egg. Before ovulation, one follicle is picked for ovulation, which is called recruitment. The egg resides in the follicle up until ovulation, when the follicle bursts. The egg is released into the fallopian tubes to be fertilized by sperm. If the egg is fertilized, it usually implants itself in the uterus within five days of being released.