What is a follicle?
An ovarian follicle is a small, fluid-filled sac in the ovary that contains one immature egg and the cells that nourish it. Each cycle, FSH recruits a group of follicles to grow. Usually one becomes the dominant follicle, reaches about 18 to 25 mm, and releases its egg at ovulation. Follicles also make most of your estrogen.
How it works in your cycle
- You're born with all the follicles you'll have, roughly 1 to 2 million, falling to about 300,000 to 400,000 by puberty.
- Early in the cycle, FSH wakes up a group of small antral follicles.
- As they grow they release estrogen; one pulls ahead and becomes dominant while the others fade.
- The LH surge makes the dominant follicle burst and release its egg: ovulation.
- The empty follicle becomes the corpus luteum, which makes progesterone.
This growth phase is why the first half of the cycle is called the follicular phase. How long a follicle takes to mature is the main reason cycle lengths differ.
Why it matters
- Fertility tests: an antral follicle count on ultrasound and AMH estimate ovarian reserve
- PCOS: many small follicles that stall before ovulation
- Fertility treatment: ultrasound tracks follicle size to time trigger shots and egg retrieval
- Cysts: a follicle that doesn't release its egg can keep growing into a harmless follicular cyst
Follicles matter for mood, too. Their rising estrogen output is behind the lift in energy and confidence many people feel in the days before ovulation.
Frequently asked questions
How big is a follicle before ovulation?
A dominant follicle is usually about 18 to 25 mm across just before it releases its egg.
Does one follicle mean one egg?
Yes, each follicle holds one egg. Occasionally two follicles release eggs in the same cycle, which can lead to non-identical twins.
Sources
This page is for information only and isn’t medical advice. If something worries you, please talk to a doctor.
Keep reading
Follicular phase: definition and meaningThe follicular phase runs from your period to ovulation, as follicles grow and estrogen rises. How long it lasts and how it tends to feel for mood and energy.
Follicle-stimulating hormone (FSH): definitionFSH is the pituitary hormone that makes ovarian follicles grow. What it does in your cycle, what high or low FSH means, and how it changes near menopause.
Ovarian reserve: definition and meaningOvarian reserve is the number of eggs left in your ovaries. How AMH and follicle counts estimate it, what it can and can't predict, and how age affects it.
Follicular phase: energy coming backWhat happens in the follicular phase: how long it lasts, why estrogen rises, the symptoms you may notice, and why mood and energy often lift.Egg (ovum): definition and meaningAn ovum is a mature egg cell released from the ovary at ovulation. How long it lives, how many you have, and how eggs change with age, in plain words.