The follicle is generally 18 to 24 millimeters at ovulation, according to Mainline Fertility & Reproductive Medicine, Ltd. It is not normally this size; it grows to this size before it releases the egg. Generally, only one follicle releases an egg each cycle.
Ultrasounds can help to determine which follicle is ready to produce an egg, according to Mainline Fertility & Reproductive Medicine, Ltd.; this process is beneficial to women trying to conceive as the doctor can predict ovulation. Ovulation generally occurs two weeks into a woman's cycle, notes the Advanced Fertility Center of Chicago, S.C.; however, the actual day varies by person, which is why an ultrasound may be needed to determine the exact date.