Acute lymphoblastic leukemia (ALL) is the most common pediatric tumor. It originates from genetic lesions in hematopoietic progenitors in fetal life. These genetic lesions have been found, in screening studies, in ~10% of healthy newborns at birth. Environmental and nutritional genotoxicants have been postulated as a risk factor but without definitive evidence. On the other hand, we have evidence that maternal environment and lifestyle affect fetal development and epigenetics. In a recent randomized clinical trial (IMPACT BCN) we have shown that structured interventions in nutrition or maternal stress improve growth and regulation of fetal cortisol.