A normal, mature follicle that is ready to ovulate and conceive is 18 to 30 millimeters in diameter. During a regular menstrual cycle, several different follicles begin to enlarge and mature, but only one follicle can complete the maturation process each month.
A follicle is a sac surrounding the egg in a human ovary. When a follicle is stimulated to mature, it enlarges and becomes filled with fluid. The egg remains attached to one wall of the follicle. Once the follicle fully matures and ovulates, the follicle ruptures and releases the egg. The egg is then captured by the fallopian tubes and travels down to the uterus.