I would say that that means that the difference is entirely structural - it's the connections between neurons that change, not the number of neurons. See this article. The number of connections doesn't necessarily increase, but the "right" connections are formed, which means you get smarter.

Also, hasn't very recent research shown that we actually can make some new neurons as adults, at least under some conditions?

Perhaps I misspoke. I was just relating what my teacher had said and I guess i misinterpreted what he meant by "brain growth". It looks like you're both right.

Neuron content aside, though, if the baby is born any later on in its physical development, the head won't fit through the pelvis. The brain being subjected to constant stimulus earlier on in its devleopment seems to be important, as intelligence is somehow related to the connections between the neurons, which are formed during the first three years after the baby is born.

Anyway, computers are definitely my specialty, so i'm really not an expert on this. But this class is just so awesome that I just had to share some of the theories with you guys. :]

(decent link showing you're both right: http://www.growingchild.com/brain.html )