It is further proposed that supra-physiological levels of estradiol and progesterone which arise from ovarian stimulation may modulate the size of the secondarily recruited follicle cohort in the next cycle. In order to explore the relative contribution of sex steroids as determinants of the size of the next cycle 'wave' of recruitment further, a second aim of this study will be investigate whether limiting the rise in sex steroid levels during ovarian stimulation, by co-treatment with aromatase inhibitor impacts on cyclic recruitment.