It is a baby from the point of the first mitosis, a unique, individual, living and very human organism. Just prior to that it is two living human cells joined together. ("Conception" is a term that denotes a creationist view, which I do not recognize as logically valid.)
This single cell at the "conception" you speak of is a zygote where the recombination of the DNA occurs, it is replaced when mitosis occurs and the DNA is replicated to facilitate the formation another identical cell. It is this transformation from zygote to embryo that is the starting point of a human life.
The societal practice of abortion is the killing something that is indeed alive and individually identifiable as a uniquely human organism.
I maintain that the societal practice of abortion is ritual mass murder upon the altars dedicated to idolatrous vanities, a collective sacrifice to pagan idols...
Suppose a lab technician takes both cells from a zygote that has just twinned and rejoins them. They then grow as though they had never separated and are implanted to yield one healthy baby.
Since, after twinning, there were two babies (by your standard) and yet the final result was that one baby was born, would you say that the lab technician had killed one? Which one? Suppose that the mitosis that produced the original twins occured with a slight mutation in a 'non-functional' part of one cell's DNA. From my understanding, this mutation would occur in about half of the cells of the resulting baby, but not in the other half. So neither twin could really be said to have "died".