The womb environment for a fetus is linked in a new study to the development of the cortex, a brain area that regulates emotions and is activated in cognitive abilities. Researchers from the Research Unit on Children’s Psychosocial Maladjustment in Quebec followed pairs of genetically identical twins from birth into their teenage years, examining the effect of different environmental factors on birth weight. They measured the twins’ weight at birth to assess whether different environmental factors had affected them while in the womb. Environmental factors that could affect birth weight include placement in the womb and access to nutrition. Previous research has identified birth weight as a predictor of the way the brain develops. In this new study, the twins share the same genetic code, thereby controlling for this factor in evaluating the role of birth weight in fetal brain development and, further, identifies the cortex as an area of the brain materially affected. Perhaps most importantly, by the time the twins reached their teen years, differences in brain volume were present in the cortex. The results reveal that what happens in the womb affects a person’s long-term brain development and that epigenetic (nongenetic) processes play a role.