Full Idea
The main sections of the brain, including the cerebral cortex, are visible within seven weeks of conception, and by the time the child is born the brain contains as many neurons - about 100 billion - as it will have as an adult.
Clarification
A 'neonate' is a new-born baby
Gist of Idea
Proper brains appear at seven weeks, and neonates have as many neurons as adults do
Source
Rita Carter (Mapping the Mind [1998], p. 17)
Book Reference
Carter,Rita: 'Mapping the Mind' [Phoenix 2000], p.17
A Reaction
Of interest in the abortion debate, and also in thinking about personal identity. However, it seems clear that the number of connections, rather than neurons, is what really matters. A small infant may well lack personal identity.