Causes of breast cancer development The main cause of breast cancer is genetic predisposition. First of all, we are talking about the presence of mutations in the BRCA1 and BRCA2 genes, although there are other genetic characteristics that can stimulate the growth of tumors. Other factors include: early onset of menstruation and late menopause late pregnancy and late delivery taking certain hormonal medications (for example, estrogens prescribed by a gynecologist) Watch the full episode of "I'm in".