PCOS on Ultrasound: What Doctors Actually See

You have just had a scan, or you are about to, and the report says something like "multiple peripherally arranged follicles, string-of-pearls appearance." That sentence panics most readers. It should not. This post translates what was actually seen, whether the count meets the current criteria, and what the word "cyst" in the report does and does not mean.

A PCOS ultrasound is not looking for cysts. It is counting antral follicles and measuring ovarian volume. The 2023 international PCOS guideline raised the follicle-count threshold to 20 or more per ovary, on transducers of 8 MHz or higher, because older machines and older thresholds were over-diagnosing the morphology in people who did not have PCOS.¹ This is one of the bigger practical changes in the 2023 update, and it means a scan you had four years ago may not be directly comparable to today's criteria.

The full picture of what is PCOS symptoms in females draws on three threads: cycles, androgens, and imaging. This post is the imaging thread. The diagnostic logic that combines all three lives in the pillar post, How PCOS Is Diagnosed: The Rotterdam Criteria Explained.

What the scan is, and how it should be done

Transvaginal ultrasound is the standard for assessing the ovaries. The probe sits at the top of the vagina, close to the ovaries, and gives a much higher-resolution image than an abdominal scan. A transabdominal scan is acceptable only when transvaginal is not appropriate, for example in someone who is not sexually active or who is very young.¹

The probe frequency matters a lot. To apply the 2023 follicle-count threshold of 20 or more per ovary, the transducer must be 8 MHz or higher.¹ Older 5 MHz probes give lower counts, and under those older probes the threshold used to be 12 or more per ovary, set by the AE-PCOS task force.² The Lujan validation work was one of the studies that drove the threshold change.³ If your scan was done on a low-resolution machine, the count it produced cannot be plugged into the 2023 criterion.

Timing also matters. The best window is the early follicular phase, cycle days 2 to 5, before any dominant follicle has emerged. In an anovulatory PCOS reader where there is no obvious follicular phase to time off, any day without a dominant follicle is acceptable. The sonographer measures both ovaries: antral follicle count, distribution of those follicles, ovarian volume, and the presence or absence of a dominant follicle or corpus luteum.

The two findings that meet the criterion

The 2023 guideline accepts either of two morphology findings:¹

1. Follicle count: 20 or more antral follicles (each measuring 2 to 9 mm) per ovary on a transvaginal scan with a probe of 8 MHz or higher.
2. Ovarian volume: 10 mL or more in either ovary, calculated excluding any dominant follicle, true cyst, or corpus luteum.

Either feature in either ovary is enough to meet the morphology criterion. Both ovaries do not need to meet it. If imaging is equivocal or unavailable at the right resolution, the 2023 guideline allows elevated AMH as an alternative: see The PCOS Blood Test Panel for what an elevated AMH looks like.¹

The Christ ultrasound-feature work showed that follicle distribution and ovarian volume correlate with the degree of reproductive and metabolic disturbance, so a meaningfully elevated ovarian volume often goes alongside a more challenging clinical picture.⁴ That is observational, not diagnostic on its own, but it explains why your clinician may pay extra attention to the volume number rather than just the follicle count.

Why "polycystic" is a misleading word

The single most important thing to understand about the report: the follicles being counted are not pathological cysts. They are antral follicles: small, fluid-filled, normal structures that every ovary contains. Every cycle, the ovary recruits a cohort of antral follicles, one becomes dominant, the rest atrophy. In PCOS the pool is larger and the recruitment-to-dominance step does not work as smoothly, so more follicles are visible at any given moment.

These follicles will not rupture. They do not need surgery. They are not pre-cancerous. The classic "string of pearls" description refers to the way these antral follicles arrange themselves peripherally, around the ovarian stroma, on imaging.⁵

A true ovarian cyst, on the other hand, is a separate finding. Simple cysts, hemorrhagic cysts, dermoid cysts, and endometriomas (chocolate cysts) are all distinct from the antral-follicle pattern of PCOS, are reported separately, and are followed up separately. If the scan report includes one of those, it has nothing to do with the PCOS morphology criterion.

Reading the report: terms to recognise

The vocabulary in a typical scan report:

• AFC: antral follicle count. The number on which the PCOS morphology criterion turns.
• Ovarian volume: calculated as length × width × height × 0.523 (the formula for a prolate ellipsoid). Reported in mL.
• Stromal hyperechogenicity: increased echogenicity of the central ovarian stroma. Descriptive only. No longer part of the formal criterion.
• Peripherally arranged follicles: descriptive, classic PCOS pattern.
• Dominant follicle: a follicle larger than about 10 mm. Suggests an ovulatory attempt is in progress.
• Corpus luteum: the post-ovulatory structure. Its presence confirms ovulation has happened in the current cycle.

If your report says "peripherally arranged follicles, multifollicular ovaries, AFC 24 left and 18 right, ovarian volume 11.2 mL left and 8.4 mL right," translate that as: 24 antral follicles in the left ovary (meets the 20-plus threshold) and ovarian volume of 11.2 mL (meets the 10-plus threshold). Either of those features is enough to satisfy the morphology criterion under the 2023 guideline.

Findings that are not PCOS but get confused with it

A scan can look "polycystic" for several reasons that have nothing to do with PCOS:

• Multifollicular ovaries in adolescents: common and physiologically normal within 8 years of menarche. The guideline does not apply the imaging criterion in that window.¹
• Multifollicular ovaries in hypothalamic amenorrhoea: can look similar on imaging but the hormonal pattern is the opposite. Low LH, low FSH, low oestradiol.
• Functional ovarian cysts (simple or hemorrhagic): a separate finding, usually resolves spontaneously over a cycle or two.
• Endometrioma: a chocolate cyst related to endometriosis. Separate diagnosis and management.
• Dermoid cyst: heterogeneous mass with fat, hair, or calcifications. Surgical follow-up.

A clinician reading your report should call out which of these, if any, are present, separately from the PCOS morphology question.

When the scan was done at the wrong time or with the wrong probe

If any of these apply, the scan result is not directly usable against the 2023 criterion:

• A transabdominal scan that reported "polycystic-looking ovaries." That report does not meet the 2023 criterion. Repeat transvaginally with a high-frequency probe before applying the criterion.¹
• A scan done mid-cycle that found a 22 mm dominant follicle. The follicle count in that setting is unreliable because the dominant follicle changes the count of small antral follicles around it. Rescan in the early follicular phase.
• A scan done within 8 years of menarche. The criterion does not apply; the language clinicians use here is "at risk of PCOS" rather than a definitive diagnosis.¹
• A scan done while on hormonal contraception. Combined oral contraceptives reduce follicle counts and ovarian volume. Wait at least three months after stopping before applying the morphology criterion.

A scan that was done with a 5 MHz probe and showed 14 follicles per ovary used to meet the older AE-PCOS criterion but does not meet the 2023 threshold of 20 with a high-frequency probe.²,³ If that is your scan, it does not mean you do not have PCOS. It means the morphology criterion was assessed under different rules and may need to be reassessed if morphology is the deciding feature in your case.

What to ask the sonographer or the clinician reviewing the report

Five questions cover most of the gaps:

1. What was the probe frequency? (5 MHz, 8 MHz, or higher.)
2. What was the antral follicle count, per ovary?
3. What was the ovarian volume of each ovary, in mL?
4. Was there a dominant follicle or corpus luteum that should have been excluded from the count or the volume calculation?
5. Were any other findings (true cysts, endometrioma, hydrosalpinx, fibroids) reported separately?

A scan report that does not contain those numbers is incomplete for PCOS assessment. You can ask for the sonographer's worksheet, which usually has the per-ovary numbers even when the formal report rounds them off.

What to do this week

Two concrete steps:

1. Pull your scan report (the patient portal, or a copy from the clinic). Check whether it states the probe frequency, the per-ovary follicle count, and the per-ovary ovarian volume. If any of those are missing, ask the clinic for the sonographer's worksheet.
2. If the scan was done transabdominally, on a low-frequency probe, within 8 years of menarche, or while you were on hormonal contraception, plan for a repeat scan under the correct conditions before the morphology criterion is applied to you.

A repeat transvaginal scan with a high-frequency probe in the early follicular phase, off the pill, is the version that the 2023 guideline expects. Most fertility clinics and many gynaecology clinics now have this equipment as standard.

What's next

• For the diagnostic framework that the scan feeds into, the pillar is How PCOS Is Diagnosed: The Rotterdam Criteria Explained.
• For the lab side of the workup, read The PCOS Blood Test Panel: What Each Number Means.
• If your scan was equivocal and AMH is being used as the alternative, read AMH, FSH, and LH Explained.
• If your symptoms fit but the morphology is borderline, read Lean PCOS: When You Don't Match the Stereotype.
• If you want the symptom-side checklist that goes with this imaging post, read PCOS Symptoms: What to Look For Before You Test.

Sources

1. Teede HJ, Tay CT, Laven JJE, Dokras A, Moran LJ, Piltonen TT, et al. Recommendations from the 2023 International Evidence-Based Guideline for the Assessment and Management of Polycystic Ovary Syndrome. Fertility and Sterility 2023;120(4):767–793. https://doi.org/10.1016/j.fertnstert.2023.07.025
2. Dewailly D, Lujan ME, Carmina E, Cedars MI, Laven J, Norman RJ, Escobar-Morreale HF. Definition and significance of polycystic ovarian morphology: a task force report from the Androgen Excess and Polycystic Ovary Syndrome Society. Human Reproduction Update 2014;20(3):334–352. https://doi.org/10.1093/humupd/dmt061
3. Lujan ME, Jarrett BY, Brooks ED, Reines JK, Peppin AK, Muhn N, et al. Updated ultrasound criteria for polycystic ovary syndrome: reliable thresholds for elevated follicle population and ovarian volume. Human Reproduction 2013;28(5):1361–1368. https://doi.org/10.1093/humrep/det062
4. Christ JP, Vanden Brink H, Brooks ED, Pierson RA, Chizen DR, Lujan ME. Ultrasound features of polycystic ovaries relate to degree of reproductive and metabolic disturbance in polycystic ovary syndrome. Fertility and Sterility 2015;103(3):787–794. https://doi.org/10.1016/j.fertnstert.2014.12.094
5. Balen AH, Laven JSE, Tan SL, Dewailly D. Ultrasound assessment of the polycystic ovary: international consensus definitions. Human Reproduction Update 2003;9(6):505–514. https://doi.org/10.1093/humupd/dmg044