Why should we believe your opposing assertion that there is no person at conception? Especially given that your assertion is motivated by a desire to kill those you deem non-persons.
I generally regret responding to idiocy.
Nor can you observe it at any other time. Which is why you lie when you imply that you accept the concept of personhood.
You are entirely motivated by the desire to justify your assertion that the tiniest human beings can be executed. Your tactics in this endeavor include demanding that a rational and scientifically justified position defend itself against your unscientific and wholly irrational assertion.
At conception, we have a new human being. Asking me to defend this assertion is like asking why water is wet except that a three year old would not be motivated by murder.
Personhood is not defined; it is conferred.
The only reason people use traits to attempt to define who is a person is to justify the exclusion of some people. Historically this has involved skin color, nationality and religion; nowadays it involves location, size, detectable brainwaves and heartbeats and the child's father's criminal history.
Whereas you make a different assertion to justify murder.
The differences between us are:
1: My claim has rational and scientific support, but most importantly,
2: I do not advocate killing.