The estrogen component of the oral contraceptives used in our study (ethinyl estradiol) is known to be 100 to 200 times stronger than estradiol, which is the estrogen generally found in circulation.41 Thus, ethinyl estradiol stimulates the epithelial compartment more strongly than estradiol does, and this action takes place through mediation by the estrogen receptors, which have greater presence in fibroadenoma than in normal tissue.36 This evidence concerns the gene ESR1 and Fibroadenoma.