The fetus of a homo sapian become a person when it take its first breath of air into its lungs. This is moment it recieves a soul. The soul being the thing that deferentiates a person from every other living thing in the Universe including homo sapian ebryos and fetuses.
The soul is the substantial form of the body. A substantial form becomes actual when a compound substance is individuated materially. A soul comes into being at individuation. Individuation occurs at fertilization.
When you understand the Aristotelian/Thomistic definitions of substance/accident, and form/matter you will be able to see more easily that your position is incoherent.
How convenient! Why do you use that as a test for personhood? Why not wait until the proto-human is able to provide for him/her self? This would give the parents a lot more time to decide whether they want to keep the little bugger. And what is this business about a soul? Why do you use theo-speek for a secular, purely rational decision?
Seriously if your going to use some arbitrary event like drawing the first breath why not use one that gives the prospective parents more freedom to choose. I mean after all that's what it's all about isn’t it? Choice, convenience, etc.