I agree with Stutefish, Christian scripture does imply that souls predate temporal bodies, but I'm just saying this to 2nd what Stutefish said, apparently you wanted a more specific event.
If you read in the account of Genesis where Adam's body was made from the dust of the earth, it wasn't until after that body was formed that "God breathed into him the breath of life", to that would imply that "life" begins at the first breath. at birth.
However, fetuses effect the exchange of gasses through the umbilicle cord and the semipermeable membrane in the placenta. Since "breathing" is the act of taking in and expelling gasses (as opposed to specifically taking air in and out through our noses), the minute a fetus begins that metabolic process could satisfy the "breathed into him the breath of life" description just as well.
For me, I believe that the spiritual definition of life is from when the soul and body join until the time they seperate... well, that pretty much tells you where I stand on the question... right? ;~D