Polycystic Ovary Syndrome (PCOS) is a common hormonal and metabolic disorder affecting women of reproductive age. It is characterized by hormonal imbalances that lead to irregular menstrual cycles and elevated male hormones. The name “polycystic” refers to numerous small, undeveloped structures on the ovaries. These are tiny follicles that have stalled in their development, not true cysts. Understanding the specific number of these follicles required for diagnosis is key to clarifying this aspect of the syndrome.
Defining the Diagnostic Threshold
The quantitative threshold for defining a polycystic ovary is highly specific and has been updated in recent international consensus guidelines for adults. Current standards define Polycystic Ovarian Morphology (PCOM) as the presence of 20 or more follicles in at least one ovary. These small structures, counted on an ultrasound, must each measure between 2 and 9 millimeters in diameter. This threshold represents a significant increase from older standards, reflecting the ability of modern imaging equipment to detect a higher number of small follicles with greater resolution.
The presence of 20 or more follicles per ovary is considered a marker of follicular excess. An alternative criterion for PCOM, often used when follicle counting is difficult, is an increased ovarian volume measuring 10 milliliters or more. Meeting either the follicle number or the volume threshold is sufficient to satisfy the PCOM component of the diagnosis.
Measuring the Count: Antral Follicle Counting
The precise measurement of these small ovarian structures is performed through Antral Follicle Counting (AFC). This technique requires the use of transvaginal ultrasound (TVUS), which provides the necessary high-resolution imaging. The proximity of the probe allows for detailed visualization, enabling the clinician to accurately count the small follicles.
New guidelines recommend using high-frequency ultrasound probes (8 megahertz or greater) to achieve the required level of detail. During the count, the sonographer systematically scans the entire ovary to count all follicles within the 2 to 9 millimeter size range. The count should ideally be performed during the early follicular phase of the menstrual cycle to standardize the measurement. It is important to ensure that the ultrasound is not performed when a dominant follicle (over 10 millimeters) or a corpus luteum is present, as these structures can obscure the view and interfere with an accurate count.
Biological Basis of Follicle Accumulation
The accumulation of numerous small follicles is a direct result of the hormonal and metabolic disruptions characteristic of PCOS. These follicles are immature and have undergone follicular arrest, meaning they start to develop but fail to reach the stage necessary for ovulation. A primary driver of this arrest is the hormonal environment, which often includes elevated levels of Luteinizing Hormone (LH) from the pituitary gland.
Elevated LH hyperstimulates the theca cells within the ovary, prompting them to produce excessive amounts of androgens (hyperandrogenism). This high concentration of androgens, combined with a relative deficiency in Follicle-Stimulating Hormone (FSH), prevents the selected follicle from maturing and releasing an egg. The follicles remain small and numerous, contributing to the characteristic appearance of the polycystic ovary.
Many individuals with PCOS also experience insulin resistance, where the body’s cells do not respond effectively to insulin. The pancreas compensates by producing higher levels of insulin (hyperinsulinemia), which acts directly on the ovaries. Insulin behaves like a co-gonadotropin, amplifying the LH-driven androgen production by the theca cells.
This excess insulin also reduces the liver’s production of Sex Hormone-Binding Globulin (SHBG), a protein that binds to and inactivates androgens in the bloodstream. The resulting increase in free, biologically active androgens further exacerbates follicular arrest. The accumulation of these small, arrested follicles also leads to high levels of Anti-Müllerian Hormone (AMH), which is thought to contribute to the ovary’s reduced sensitivity to FSH, locking the follicles in their immature state.
Placing the Follicle Count in the Overall PCOS Diagnosis
While the follicle count is a distinct and measurable feature, it is only one piece of the puzzle in diagnosing PCOS. The diagnosis is typically made using the widely accepted Rotterdam criteria, which require a patient to meet two out of three specific conditions:
1. Oligo- or anovulation (irregular or absent menstrual cycles).
2. Clinical or biochemical hyperandrogenism (signs like excess hair growth, acne, or elevated androgen blood levels).
3. Polycystic Ovarian Morphology (PCOM) as determined by the follicle count or ovarian volume.
A high follicle count alone is not sufficient for a PCOS diagnosis. Many women without the syndrome may show a high number of follicles on an ultrasound without experiencing hormonal symptoms or irregular cycles. A healthcare provider must evaluate the complete clinical picture and ensure that other conditions that might mimic PCOS symptoms are excluded before confirming the diagnosis. Serum Anti-Müllerian Hormone (AMH) levels can also be used as an alternative measure for PCOM in adults, but this must be combined with one of the other two criteria for a definitive diagnosis.