Mary Anne,

It is a dogma of the faith that the Second Person of the Trinity, God the Word, became incarnate from the first moment of His Conception. 

This is demonstrated, among other things, by the fact that John the Baptist leapt in the womb of St. Elizabeth when Mary visited her. (Luke 1:41-44)

Consequently, it would be heretical to assert that Jesus became man, at a later time, than His Conception, viz., His Birth.

Eric

Mary Anne,

No. However, this is how I would say it: The eternal Son took on a human nature at His Conception.

Yours is looser and more general than mine, but it's fine. I like to emphasize the moment God became man, which was nine months before His Birthday.

The important thing is that we convey the unfathomable truth that the second Person of God, the Trinity, took on a human nature to save humanity from the deadly effects of their sins, whom Mary and Joseph named Jesus. 

Peace,

Paul