Follicle number per ovary (FNPO) of 20 or more on a transvaginal scan with a high-frequency probe, or an ovarian volume of 10 mL or more — one criterion of three for PCOS, never the diagnosis on its own.
Orient first
- Rotterdam criteria: two of oligo/anovulation, hyperandrogenism, polycystic ovarian morphology (verify the 2023 international guideline thresholds in use).
- The follicle threshold depends on the probe frequency; older 12-follicle criteria are obsolete.
- Adolescents within 8 years of menarche should not be labelled on ultrasound.
Acquire the study
- Transvaginal probe of ≥ 8 MHz in the early follicular phase (cycle day 2–5) or at random in amenorrhoea; no dominant follicle > 10 mm.
The manoeuvre
- Each ovary in three planes: volume in mL (0.52 × length × width × depth in cm).
- Sweep the whole ovary and count follicles 2–9 mm in diameter (FNPO).
- Note a dominant follicle or corpus luteum — repeat in the next cycle if present.
- Stroma: increased stromal echogenicity is supportive only.
- Endometrium thickness in mm (unopposed oestrogen).
What confirms it
- FNPO ≥ 20 in at least one ovary (or volume ≥ 10 mL) on a compliant transvaginal scan — reported as "polycystic ovarian morphology".
What licenses you to exclude it
- FNPO below threshold and volume below 10 mL in both ovaries exclude the morphology; they do not exclude PCOS.
The classic misread
- Writing "PCOS" in the impression — the diagnosis is clinical.
- Counting follicles on a transabdominal scan (undercounts).
- Measuring volume with a dominant follicle present.