An ovarian follicle is a small, fluid-filled sac in the ovary that contains a developing egg. It’s the fundamental functional unit of the ovary, responsible for both maturing eggs and producing the hormones that drive the menstrual cycle. A follicle is not the egg itself. Think of it as a protective shell: layers of specialized cells wrapped around a single egg, nourishing it and responding to hormonal signals until it’s ready for ovulation.
Follicle Structure
At its simplest, a follicle is an egg surrounded by a single flat layer of support cells called granulosa cells. As the follicle matures, those granulosa cells multiply from one layer into many, and a second outer layer called the theca develops around them like a capsule. These two cell types work as a team to produce hormones. The theca cells produce androgens, and the granulosa cells convert those androgens into estrogen. After ovulation, the leftover granulosa cells switch roles and begin producing progesterone instead.
As the follicle grows larger, fluid accumulates between the granulosa cells and eventually pools into a central cavity called the antrum. At this point, the egg sits off to one side, nestled in a mound of granulosa cells. When the follicle finally ruptures during ovulation, the egg is released along with a ring of protective cells surrounding it, and these travel together into the fallopian tube.
How a Follicle Develops
Follicle development, called folliculogenesis, is a remarkably slow process. It takes close to a full year for a resting follicle to grow from its dormant state to the point of ovulation. The journey happens in distinct stages, each defined by physical changes in the follicle’s size and structure.
The earliest form is the primordial follicle, a tiny structure about 25 micrometers across (roughly a quarter the width of a human hair). It contains an immature egg surrounded by a single flat layer of granulosa cells. Women are born with their entire supply of these primordial follicles, and they sit dormant until recruited into the growth process.
When a primordial follicle is activated, its granulosa cells change shape from flat to cuboidal, making it a primary follicle. As those cells multiply into two or more layers, it becomes a secondary follicle, which also gains its outer theca layer. This entire preantral phase takes roughly 300 days.
The next milestone is cavitation, when fluid begins to collect inside the follicle and form the antrum. The follicle is now an antral follicle (also called a Graafian follicle), and it enters the phase visible on ultrasound. Antral follicles range from about 1 mm to over 20 mm depending on their stage of growth:
- Small antral: 1 to 6 mm
- Medium: 7 to 11 mm
- Large: 12 to 17 mm
- Preovulatory: 18 to 23 mm
A selected dominant follicle needs about 15 to 20 days to reach the ovulatory stage. Just before ovulation, the average dominant follicle measures 22 to 24 mm in diameter, though anything from 18 to 36 mm falls within the normal range.
Hormones That Drive the Process
Two pituitary hormones orchestrate follicle growth: follicle-stimulating hormone (FSH) and luteinizing hormone (LH). At the start of each menstrual cycle, rising FSH stimulates a group of antral follicles to grow. Of that group, typically only one becomes the dominant follicle, while the rest stop growing and are reabsorbed.
LH has traditionally been considered important only in the later stages of follicle development, but newer evidence shows that functional LH receptors appear even in very small, early-stage follicles. LH appears to help accelerate the transition of follicles from early growth stages into the antral stage, which may influence how many follicles are available for recruitment each cycle. This helps explain why two women with similar numbers of resting follicles can have different numbers of antral follicles visible on ultrasound.
The growing follicles themselves produce estrogen, which rises steadily in the first half of the cycle. When estrogen reaches a critical level, it triggers a surge of LH from the pituitary, and that surge causes the dominant follicle to rupture and release its egg.
What Happens After Ovulation
Once the dominant follicle ruptures, it doesn’t simply disappear. The remaining granulosa and theca cells reorganize into a new structure called the corpus luteum, a yellowish mass of cells that seals the break where the egg escaped. The corpus luteum produces estrogen and, critically, progesterone. Progesterone prepares the uterine lining to support a potential pregnancy.
If fertilization occurs, a hormone from the early embryo (hCG, the same hormone detected by pregnancy tests) signals the corpus luteum to keep producing progesterone. It continues doing so for about 12 weeks, at which point the placenta takes over hormone production. If no pregnancy occurs, the corpus luteum breaks down after about two weeks, progesterone drops, and menstruation begins.
Most Follicles Never Ovulate
The vast majority of follicles a woman has will never release an egg. Of the roughly one to two million primordial follicles present at birth, only about 400 will ever fully mature and ovulate during the reproductive years. That means 99.9% of follicles undergo a natural process called atresia, where they stop developing and are gradually reabsorbed by the body. Even within a single cycle, when a group of antral follicles begins growing, 99% of them will undergo atresia. Only the single dominant follicle (or occasionally two) completes the journey to ovulation.
This constant attrition is why the follicle supply declines with age. The rate of loss isn’t constant. Before age 37, the number of antral follicles drops by an average of 4.8% per year. After 37, that rate more than doubles to 11.7% per year, which is one reason fertility declines more sharply in the late thirties.
Follicle Counts and Fertility
Because follicles are the functional units that contain eggs, counting them provides a window into a woman’s remaining egg supply, often called ovarian reserve. An antral follicle count (AFC) is done via transvaginal ultrasound, which measures the number of small antral follicles (2 to 9 mm) visible in both ovaries. A higher count generally suggests a larger remaining pool of eggs.
A blood test for anti-Müllerian hormone (AMH) offers another angle on the same question. AMH is produced by the granulosa cells of primary, preantral, and early antral follicles. Because it comes from these small, growing follicles rather than from the dominant follicle, AMH levels stay relatively stable throughout the menstrual cycle, making it a convenient marker to test at any point. As the follicle pool shrinks with age, AMH levels decline in parallel.
Follicles and Polycystic Ovary Syndrome
In polycystic ovary syndrome (PCOS), the ovaries contain an unusually high number of small antral follicles. Despite the name, these aren’t true cysts. They’re follicles that have started growing but stalled before reaching the dominant stage, so they accumulate in the ovaries rather than progressing to ovulation.
The current diagnostic threshold for polycystic ovarian morphology is 20 or more follicles (measuring 2 to 9 mm) in at least one ovary on ultrasound, or an ovarian volume of 10 ml or greater. This follicle pattern is just one of the criteria used to diagnose PCOS and doesn’t on its own confirm the condition. Many women with this ovarian appearance on ultrasound have no other symptoms.