The first window of opportunity is 8 - 13 weeks after fertilization and the referent is the mother's diet.
If nothing else, considering that at my age, per day (on a good day) it is estimated that I lose 10,000 neurons, it would have been nice to have started out with a good cache, say at least 100 billion.
I cringe at the thought of the many cups of coffee, and Twinkies (cupcakes) and Martinis my mother consumed before she learned she was pregnant.
As an interesting side note, however, scientists have recently discovered that Nature has left the window for new neural cell development slightly ajar.
In the area of the brain called the hippocampus, which plays a major role in learning, a small number of new cells are created even in later life.
Notes: New brain cell generation in adults