This is one definition.
Others exist.....
- It becomes a human as soon as it is viable outside the womb.
- It becomes a human as soon as it is born.
- It becomes a human after x weeks gestation. (The value of x is a subject of contention.)
Why is your definition the only valid one?
Because it is the one supported by rock solid science. Human beings begin at conception and end at death. This is as unambiguous as the earth orbiting the sun.
Tom