AFC

What it is

The AFC (antral follicle count) is the total number of 2 to 9 mm follicles counted across both ovaries on transvaginal ultrasound. These small antral follicles are at the stage where they become visible on ultrasound but before being recruited for ovulation.

The AFC is both an ovarian reserve marker (a higher count indicates a more abundant reserve) and a PCOS diagnostic criterionas a component of the PCOM (polycystic ovarian morphology) criterion.

Updated PCOS threshold (2023)

According to the ESHRE 2023 guidelines, the PCOM criterion is defined by an AFC of ≥ 20 follicles of 2-9 mm per ovary. This threshold replaces the old ≥ 12 follicles per ovary threshold established at the 2003 Rotterdam consensus.

Note that some centres and ultrasound reports express the AFC differently (per ovary or as a total for both ovaries). The ≥ 20 threshold applies per ovary; if the total AFC is reported, the corresponding threshold would be ≥ 40 — but in practice, the per-ovary threshold is used in diagnostic criteria.

AFC and ovarian reserve

Outside of PCOS, the AFC is used to assess ovarian reserve before ovarian stimulation (IVF, IUI):

• Low AFC (< 7 total): diminished reserve, reduced ovarian response
• Normal AFC (7 – 19 total): normal reserve
• High AFC (≥ 20 per ovary): abundant reserve — in the context of PCOS, risk of ovarian hyperstimulation syndrome (OHSS) during IVF

Relationship with AMH

The AFC and AMH both reflect the same follicle pool. They are strongly correlated: a high AFC is generally accompanied by a high AMH. The two markers are complementary — AMH can be measured at any point in the cycle by a simple blood draw, whereas AFC requires an ultrasound scan.

The ESHRE 2023 guidelines note that an AMH threshold can, with calibrated laboratories, replace the AFC for the PCOM criterion. This remains subject to local validation.

Variability and limitations

The AFC is operator-dependent and depends on the quality of the ultrasound probe. Studies show inter-observer variability of ± 15 to 25%. A single AFC result should therefore not be over-interpreted, especially at borderline values (18-22 follicles).

Key takeaways

• Sum of 2-9 mm follicles across both ovaries on transvaginal ultrasound
• PCOM criterion (PCOS): ≥ 20 follicles per ovary (ESHRE 2023)
• Ideal timing: cycle days 2 – 4
• Strongly correlated with AMH — both reflect the follicle pool
• Operator variability ± 15-25% — do not over-interpret borderline values