The number of small antral follicles (2–6 mm) determines the outcome of endocrine ovarian reserve tests in a subfertile population

BACKGROUND: Ovarian reserve is related to age and can be estimated by ovarian reserve tests (ORTs), such as antral follicle count (AFC) and various endocrine parameters. The endocrine function of a follicle is related to its size. The aim of this study is to evaluate which sizes of antral follicles are most closely correlated with age and the outcome of endocrine ORTs. METHODS: In total 474 subfertile, ovulatory patients, recruited from two fertility centers in The Netherlands, participated in this prospective cohort study. The following ORTs were performed: AFC (follicles from 2 to 10 mm), basal FSH, basal inhibin B (bInhB), clomiphene citrate challenge test and inhibin B after stimulation with clomiphene citrate. RESULTS: The number of small follicles (2-6 mm) declined with age; the number of larger follicles (7-10 mm) remained constant. Independent of age, the number of small follicles was significantly related to all ORTs (P <0.001, except bInhB P = 0.005). The number of larger follicles was only significantly related to bInhB (P = 0.009). CONCLUSIONS: The number of small antral follicles (2-6 mm) is significantly related to age and also, independent of age, to all endocrine ORTs tested, suggesting the number of small antral follicles represents the functional ovarian reserve.