An ovarian follicle is a small, fluid-filled sac in the ovary that contains a developing egg. Each month, follicles grow and compete until one releases a mature egg during ovulation. Follicles also function as hormone-producing structures, making the estrogen and progesterone that drive your menstrual cycle and support early pregnancy.
What a Follicle Is Made Of
Each follicle has three main components. At the center is a single oocyte, the immature egg cell. Surrounding the egg is a layer of granulosa cells that nourish it and produce hormones. On the outside sits a shell of theca cells, which supply blood and produce the raw hormonal ingredients that granulosa cells convert into estrogen.
Think of it as a protective capsule designed to feed and mature one egg at a time. The granulosa cells act like support staff, responding to hormonal signals from the brain and converting androgens from the theca cells into estradiol, the primary form of estrogen. This cooperative hormone production is central to how follicles drive your entire cycle.
How Follicles Develop Over Time
A follicle takes roughly a year to go from its dormant state to the point of ovulation. Most of that time is spent in early, slow growth that doesn’t depend on hormonal signals from the brain. Only in the final weeks does the process accelerate under the influence of FSH and LH, the two key reproductive hormones released by the pituitary gland.
Early Stages
Every follicle starts as a primordial follicle: a tiny, sleeping structure with an immature egg surrounded by a single flat layer of cells. When activated, the granulosa cells multiply and become cube-shaped, forming a primary follicle. At this stage, the follicle develops FSH receptors, essentially installing an antenna that lets it receive growth signals later on.
As the primary follicle grows into a secondary follicle, its cell layers thicken, it develops its own blood supply, and the theca cell layer forms on the outside. The theca cells develop LH receptors, giving the follicle a second antenna for hormonal communication.
The Antral Stage
Once FSH from the brain kicks in, fluid begins to accumulate inside the follicle, creating a cavity called an antrum. This is the antral stage, and it’s the point at which follicles become visible on ultrasound, typically measuring 2 to 10 millimeters. Several antral follicles grow simultaneously each cycle, competing for dominance.
Dominant Follicle Selection
Out of the group of growing antral follicles, one is selected as the dominant follicle. This winning follicle shifts from depending on FSH to relying primarily on LH for its final growth spurt. It swells rapidly, reaching about 18 to 24 millimeters. Its granulosa cells develop high concentrations of LH receptors, priming it for the hormonal surge that triggers ovulation. The remaining follicles, now outcompeted, stop growing and break down.
What Happens at Ovulation and After
A surge of LH from the pituitary gland causes the dominant follicle to rupture, releasing the mature egg into the fallopian tube. The empty follicle doesn’t disappear. Instead, it transforms into a structure called the corpus luteum, which pumps out progesterone and estrogen to prepare the uterine lining for a potential pregnancy.
If the egg isn’t fertilized and implantation doesn’t occur, the corpus luteum degenerates after about two weeks into a small scar-like remnant called the corpus albicans. Progesterone drops, the uterine lining sheds, and your period begins.
Most Follicles Never Ovulate
You’re born with roughly 1 million follicles in your ovaries. By puberty, about 500,000 remain. By your late 30s, the number drops to somewhere between 10,000 and 50,000. Eventually the pool runs out entirely, and menopause begins.
The vast majority of those follicles, about 99.9%, never release an egg. They undergo a natural process called atresia, a form of programmed cell death that breaks the follicle down and reabsorbs it. Over an entire reproductive lifetime, only about 400 to 500 follicles will successfully ovulate. Atresia isn’t a malfunction. It’s how the ovary selects only the healthiest eggs and maintains its normal function.
Follicle Count and Fertility
When doctors assess fertility, one of the first things they check is your antral follicle count (AFC), the number of small follicles visible on a transvaginal ultrasound early in your cycle. These follicles measure 2 to 10 millimeters and represent the pool of eggs available for that cycle and, more broadly, your remaining ovarian reserve.
A combined count from both ovaries of 10 or more is generally associated with a good chance of natural conception. In one study comparing fertile and infertile women, 86.7% of women with proven fertility had an AFC above 10, compared to only 40% of women presenting with infertility. The median count for fertile women was 13, while for infertile women it was 9. A count below 5 to 7 typically signals a diminished ovarian reserve, meaning fewer eggs are available to respond to hormonal stimulation.
AFC naturally declines with age, which is why fertility specialists interpret these numbers relative to how old you are rather than using a single universal cutoff.
When a Follicle Becomes a Cyst
Sometimes a follicle doesn’t rupture as expected. Instead of releasing its egg, it keeps growing and filling with fluid. This creates a follicular cyst, the most common type of ovarian cyst. Similarly, after ovulation, the corpus luteum can sometimes seal off and accumulate fluid, forming a corpus luteum cyst.
Both types are called functional cysts because they arise from normal follicle activity. They’re usually harmless and resolve on their own within two to three menstrual cycles without treatment. Larger cysts can cause a dull or sharp pain on one side of the lower abdomen, bloating, or a feeling of pressure. The bigger a cyst grows, the greater the chance it could rupture, which can cause sudden, sharp pain.
Follicles and Polycystic Ovary Syndrome
In PCOS, the ovaries contain an unusually high number of small follicles that stall in early development and don’t progress to ovulation. Despite the name “polycystic,” these aren’t true cysts. They’re immature follicles stuck at the antral stage, typically 2 to 9 millimeters in size.
The diagnostic ultrasound criteria established at a 2003 consensus conference define polycystic ovary appearance as 12 or more of these small follicles per ovary, or an ovarian volume greater than 10 milliliters. Updated research suggests these thresholds should be adjusted by age: for women under 24, a count of 13 or more per ovary is the cutoff, while for women over 44, as few as 7 follicles per ovary may qualify. The high follicle count in PCOS reflects disrupted signaling rather than extra fertility. Without proper dominant follicle selection, ovulation becomes irregular or absent, which is the primary reason PCOS affects conception.