To you, and that's the biggest problem. The inverse of what was stated later applies: You need to get in through your skull that the other side has a logically coherent, even if not fully fleshed out to the last edge-case, idea of when life starts that draws the line later than yours.
Using conception doesn't work, because then 90% of humans are miscarried. Probably even more. Extremely few zygotes make it to term. It makes life insultingly cheap, because the extremely vast minority of the time it goes nowhere at all.
Your own definition also fails to cover identical twins and cloning, because we land right back at featural definitions to define when the lives become separate. In which case, why not use such in the first place instead of putting moral value on something with less than a 10% chance of getting any meaningful biological features?