BDNF and depressive disorder: Stressful conditions in pregnancy also due to poverty and low social status, as well as anxiety, depression and poor nutrition correlate with epigenetic alterations in the fetus and affect key molecules and systems: the neuroendocrine stress axis (with the NR3C1 gene coding for the cortisol receptor), the serotonin circuit (with alteration of the gene coding for the serotonin transporter, SLC6A4), the oxytocin circuit (OXTR) and the brain plasticity circuit (BDNF).