The present study tests the hypothesis that a critical window for cortical development coincides with the period of neural stem cell proliferation (during the first 6 weeks of gestation), specifically, gastrulation (on embryonic day [E] 19 and E20). Pregnant female macaques were exposed to ethanol 1 day/week for 6 or 24 weeks such that it included E19 or E20 or a time before or after the time of gastrulation. Total forebrain size was increased in macaques exposed to ethanol on E19 or E20. Thus, various features of the gray and white matter of the paracentral lobule of adolescent offspring were examined. Ethanol exposure affected the gray matter, for example, the 1.63 billion neurons in somatosensory cortex of controls (areas 3a and 3b) was 32% lower in ethanol-exposed monkeys, but neither duration nor timing of the episodic exposure had a differential effect. In contrast, the timing of the exposure during the third week critically affected the amount of white matter (the mass of myelopil, but not cell number). Therefore, fetal exposure to ethanol unveils a normal programming mechanism wherein neural stem cells appear to be a target and a critical window for forebrain development concurs with gastrulation.