We previously reported a mutation in the cholesterol biosynthesis gene, hydroxysteroid (17-beta) dehydrogenase 7 (Hsd17b7^rudolph), that results in striking embryonic forebrain dysgenesis. Here we describe abnormal patterns of neuroprogenitor proliferation in the mutant forebrain, namely, a decrease in mitotic cells within the ventricular zone (VZ) and an increase through the remainder of the cortex by E11.5. Further evidence suggests mutant cells undergo abnormal interkinetic nuclear migration (IKNM). Furthermore, intermediate progenitors are increased at the expense of apical progenitors by E12.5, and post-mitotic neurons are expanded by E14.5. In vitro primary neuron culture further supports our model of accelerated cortical differentiation in the mutant. Combined administration of a statin and dietary cholesterol in utero achieved partial reversal of multiple developmental abnormalities in the Hsd17b7^rudolph embryo, including the forebrain. These results suggest that abnormally increased levels of specific cholesterol precursors in the Hsd17b7^rudolph embryo cause cortical dysgenesis by altering patterns of neurogenesis.