The current international guideline sets the threshold at 20 or more follicles per ovary for a polycystic ovary morphology (PCOM) diagnosis in adults. That said, the number has changed over time, and the follicle count alone doesn’t determine whether you have PCOS. Here’s what the numbers actually mean and how they fit into the bigger diagnostic picture.
The Follicle Count Threshold
What doctors look for on ultrasound are not true cysts but small, fluid-filled follicles measuring 2 to 9 mm in diameter. Each one contains an immature egg that started developing but stalled before ovulation. The name “polycystic” is misleading because these follicles behave differently from the larger, fluid-filled cysts that can form on ovaries for other reasons.
The 2023 international evidence-based guideline, the most current standard, recommends a threshold of 20 or more follicles in at least one ovary when using modern ultrasound equipment. This replaced the older Rotterdam criteria from 2003, which set the bar at 12 or more follicles per ovary. The original number of 12 came from a study showing it provided 99% specificity in distinguishing women with PCOS from those without it, but ultrasound technology has improved dramatically since then. Higher-resolution machines pick up more small follicles in healthy ovaries, which made the old cutoff too easy to meet. The Androgen Excess Society previously suggested raising the number to 25, and the current guideline landed at 20 as the updated standard.
If the ultrasound equipment is older or image quality is poor, clinicians can use an alternative measure: an ovarian volume of 10 mL or greater, or 10 or more follicles per cross-section of the ovary. Some researchers have argued the volume threshold should be lowered to 7 mL, which in one study offered 95% specificity and 69% sensitivity for identifying polycystic ovaries.
Why Follicles Accumulate in PCOS
In a typical menstrual cycle, several follicles begin growing each month, but one becomes dominant and releases an egg. In PCOS, excess androgens (hormones like testosterone) disrupt this process at a cellular level. High testosterone triggers stress and self-destruction in the granulosa cells that surround and nourish each developing egg. It also suppresses the genes responsible for the final stages of follicle maturation and ovulation.
The result is that follicles start developing but never reach the point of releasing an egg. Instead of one follicle completing the journey each cycle, many stall at a small size and linger. Over time, this creates the characteristic “string of pearls” appearance on ultrasound, with dozens of tiny follicles lined up along the outer edge of each ovary. The ovaries often enlarge as well, which is why volume is used as a backup measurement.
You Can Have PCOS Without Polycystic Ovaries
The follicle count is only one of three diagnostic criteria. PCOS is diagnosed when you meet at least two of the following three:
- Irregular or absent periods, which signal that ovulation isn’t happening regularly
- Excess androgens, either measured through a blood test or visible as acne, excess facial hair, or hair thinning on the scalp
- Polycystic ovary morphology on ultrasound
This means you can be diagnosed with PCOS without ever having an ultrasound. If you have irregular periods and elevated testosterone on a blood test, that’s enough. Conversely, having polycystic-looking ovaries on ultrasound doesn’t automatically mean you have PCOS. Up to 20-30% of women in the general population have ovaries that meet the morphology criteria without having the syndrome.
Why Ultrasound Is Skipped for Teens
Adolescent girls are a special case. The Pediatric Endocrine Society recommends that pelvic ultrasound generally be deferred during PCOS evaluation in teenagers. The reason is straightforward: irregular periods, acne, and ovaries with many small follicles are all normal features of puberty. Ovaries in healthy teens naturally contain a high number of follicles, making the adult thresholds unreliable. Diagnosing PCOS in this age group relies more heavily on persistent irregular cycles (beyond what’s expected in the first few years after a first period) and clear evidence of excess androgens.
What the Number Means for You
If you’ve had an ultrasound and are trying to interpret the results, the key details to look at are the follicle count per ovary and the ovarian volume. A count of 20 or more follicles (2 to 9 mm each) in either ovary, or a volume of 10 mL or more, meets the current morphology criteria. But remember this is just one piece of the puzzle. The follicle count tells your doctor that your ovaries look a certain way. It doesn’t tell them why, and it doesn’t confirm PCOS on its own.
It’s also worth knowing that follicle counts can vary depending on where you are in your cycle, how full your bladder is, whether the ultrasound is done abdominally or transvaginally, and the skill of the person performing it. Transvaginal ultrasound is more accurate for counting small follicles. If your results are borderline, a repeat scan or additional blood work can clarify things.