At approximately day 1

At birth, the brain is about 800 g.

Reference

Evans OB, Hutchins JB. 1997. Development of the Nervous System. Chapter 5 In: Haines DE, ed. Fundamental Neuroscience. New York: Churchill Livingstone.

At approximately day 1

At birth, the brain is approximately 25% of its adult volume; most postnatal growth is due to myelination.

Reference

Larsen WJ. 1997. Human Embryology. New York: Churchill Livingstone Inc.