Brain development requires a massive increase in brain lipogenesis and accretion of the essential omega-3 fatty acid docosahexaenoic acid (DHA). Brain acquisition of DHA is primarily mediated by the transporter Major Facilitator Superfamily Domain containing 2a (Mfsd2a) expressed in the endothelium of the blood-brain barrier. Mfsd2a transports DHA and other polyunsaturated fatty acids esterified to lysophosphatidylcholine (LPC-DHA). However, the function of Mfsd2a and DHA in brain development is incompletely understood. Using vascular endothelial-specific (2aECKO) and inducible vascular endothelial-specific (2aiECKO) deletion of Mfsd2a in mice, we found Mfsd2a to be uniquely required postnatally at the blood-brain barrier for normal brain growth and DHA accretion, with DHA deficiency preceding the onset of microcephaly. Gene expression profiling analysis of these DHA deficient brains indicated that Srebp-1 and Srebp-2 pathways were highly elevated.