Full term pregnancy at an early age is the only factor known to consistently protect against breast cancer. Because hormone receptor positive progenitors in the human breast relay endocrine signaling, we here sought to determine whether an experimental mimicry of the third trimester surge of hormones would change their susceptibility to growth stimulation. Hormone receptor positive, reduction mammoplasty-derived human breast epithelial progenitors were exposed to a short-term, pregnancy-level of estradiol, and their subsequent response to estradiol stimulation was analyzed. Exposure to pregnancy-level of estradiol results in subsequent lower sensitivity to estrogen-induced proliferation. Expression array and immunoblotting reveal upregulation of S100A7 and down-regulation of p27, both associated with parity and epithelial differentiation. Notably, we find that the epithelial differentiation is accompanied by upregulation of E-cadherin and down-regulation of vimentin as well as by diminished migration and more mature luminal epithelial differentiation in a mouse transplantation model. Our findings are in support of a de-sensitization mechanism for pregnancy-induced prevention against breast cancer.