Pelvic Ultrasound
Ovarian imaging and PCOM criterion in PCOS diagnosis
What it is
A pelvic ultrasound is an imaging examination that uses sound waves to visualise the ovaries, uterus, and pelvic structures. In the context of PCOS, it is used to assess ovarian morphology — specifically the number of small visible follicles and the size of the ovaries.
This is the third Rotterdam criterion (criterion 3), designated PCOM (polycystic ovarian morphology). On its own, this criterion is not sufficient to diagnose PCOS: at least 2 of 3 Rotterdam criteria must be met.
PCOM diagnostic thresholds (updated 2023)
The ESHRE 2023 guidelines updated the thresholds used to define PCOM:
- Antral follicle count ≥ 20 follicles of 2-9 mm per ovary — new threshold (replaces the old ≥ 12 follicle threshold established in 2003)
- Ovarian volume ≥ 10 mL — complementary measure, may be sufficient if follicle counting is difficult
The rise in threshold from 12 to 20 follicles reflects advances in ultrasound technology: modern probes detect smaller follicles with greater resolution.
Probe choice and timing
The transvaginal route is preferred because it provides better resolution of pelvic structures and allows more accurate follicle counting. The transabdominal route is used in individuals who have not had penetrative sex or who decline vaginal examination — it is less precise.
The scan is ideally performed in the early follicular phase (cycle days 2 to 5), when the ovaries are not affected by a dominant follicle or recent ovulation. In the absence of periods (amenorrhoea), it can be performed at any time.
Limitations of pelvic ultrasound
Pelvic ultrasound is technology-dependent (probe quality, counting software) and operator-dependent. The same ovary may yield different follicle counts depending on the radiologist or equipment. A single scan should not be over-interpreted.
Furthermore, polycystic ovarian morphology can be found in women without PCOS (up to 20% of women of reproductive age). This is why diagnosis cannot rest on ultrasound alone.
Key takeaways
- Rotterdam criterion 3: PCOM = ≥ 20 follicles 2-9 mm per ovary OR volume ≥ 10 mL
- Threshold updated in 2023 (old threshold: ≥ 12 follicles)
- Transvaginal probe preferred — transabdominal if no penetration or refusal
- Ideal timing: cycle days 2 – 5
- Technology- and operator-dependent result
- Insufficient alone to diagnose PCOS (2 of 3 criteria required)