As estrogen is the main driver of the histological changes of the breast during pregnancy,32 the increased proportion of hormone-independent tumours in the later trimesters could reflect an indirect effect of estrogen on promoting tumour growth independent of ER,33 and a potential pro-oncogenic transition of existing ER/PR-negative breast cancer stem-like cells.29, 30, 31. This evidence concerns the gene ESR1 and breast carcinoma.