Prospective analysis of the relationships between the ovarian follicle cohort and basal FSH concentration, the inhibin response to exogenous FSH and ovarian follicle number at different stages of the normal menstrual cycle and after pituitary down-regulation

BACKGROUND: Analyses of the follicular reserve and activity of the ovary are central to our understanding of the regulation of follicular development. We have carried out a prospective analysis of endocrine and biophysical assessments under three differing basal conditions: the early follicular and mid-luteal phases, and following GnRH analogue down-regulation. METHODS: Hormonal analyses were carried out before and after a single dose of FSH on spontaneously ovulating women (n = 58). Ovarian volume and antral follicle count (AFC) were also determined. RESULTS: Inhibin B and estradiol concentrations were increased by FSH under all three conditions, and inhibin A in the follicular phase and after down-regulation. Basal hormone concentrations, except inhibin A and B after down-regulation, did not generally correlate with AFC. A close relationship between inhibin B and AFC was evident at all stages after FSH administration (r = 0.70-0.77). AFC and inhibin B after FSH stimulation were well correlated with the number of oocytes recovered after superovulation. Multivariate analysis demonstrated that inhibin B after FSH administration in the down-regulated state showed the closest correlation with oocyte number. In the more clinically useful early follicular and luteal phases, basal FSH was the most significant contributor to the number of oocytes, with a significant contribution from luteal phase AFC. CONCLUSIONS: These data extend our understanding of the relationships between follicular number, follicular functional activity, and the recruitable follicular population. Down-regulation and subsequent FSH stimulation was required to clearly demonstrate the close relationship between inhibin B and the ovarian reserve. Without such complex manipulation, early follicular phase FSH (supplemented by AFC in the relatively hypogonadotrophic luteal phase) remains of greater value in predicting the ovarian reserve than the currently known direct products of the ovary.