Pyrethroid insecticides are commonly used in agriculture and household products. Prenatal exposure to pyrethroids has been associated with a variety of adverse neurodevelopmental outcomes in children, including deficits in motor control, social adaptation, and intelligence. Cypermethrin, a type II pyrethroid, is the most frequently detected pyrethroid in food. Preliminary data suggest that prenatal exposure to cypermethrin alters the differentiation and migration of GABAergic interneurons in the embryonic mouse brain at gestational day 14. To further investigate the effects of cypermethrin on the development of GABAergic interneurons, CD1 dams were treated with 0, 3mg/kg, or 10mg/kg cypermethrin from gestational days 5 to 18 via oral gavage. GAD67-GFP+ embryonic brains were fixed with 4% PFA, sectioned at a thickness of 25 microns, and stained with DAPI. Unbiased stereological cell counts of GABAergic interneurons in the white matter, a major migratory pathway, were obtained using the Optical Fractionator probe. To assess global neurodevelopment, cortical volume in the embryonic brain was measured using 5-6 serial sections; 20 sections apart. In conclusion, 10mg/kg Cypermethrin from gestational days 5 to 18 caused a significant increase in GABAergic cell density in the white matter of the embryonic brain at E18, a developmental time point when migrating cells should be integrating into the gray matter.