Look up "Life cycle of a placental mammal". It's very basic life science.
After that, the question is "Which human beings are persons?" History is littered with tragedies resulting from some human beings deciding that other human beings are not persons, and therefore expendable.
Tom
Still, we will wrestle with defining when life as a separate person begins.
No matter how much scientific progress we see, the definition will be arbitrary.
Some will say at conception. Others will say at birth. Many will be in between.
The beginning will be a definition based upon compromise. It won't be true,
just a convenient & uneasy consensus.
I like a perspective which was presented by an anti-abortion friend.
(It troubled him, & made it difficult to be anti-abortion. An interesting guy.)
Premise: One should not be forced to enable life for another person.
No one shouldn't have to give up an organ (eg, kidney) or otherwise
have one's body appropriated.
From the above premise, the right to have an abortion would be deduced.
The fetus, whether a person or mere precursor, would have rights
subservient to the mother, who has the right to end support for the fetus.
There are still vexing complexities to address....
1) Will society make this right a "window", eg, limiting abortion to the 1st 2/3 of gestation?
This would give ample opportunity for exercising the right to abortion, while avoiding
the possibly general objection to late term abortion of what many would consider a
"baby", ie, a person. This seems a reasonable compromise.
2) The mother would have right to terminate the pregnancy, but would have limitations
upon what she can do to her own body when it affects another person. Looking at a
longer time frame, if the mother did something which imposes damage upon the future
person (a fetus which is carried to term, born, & matures), this might be regulated.
Caution:
Before anyone fulminates at my bigoted, sexist, anti-feminist intersectional privileged ravings,
remember that this is only a perspective. I don't say that it's "true". Tis only to be considered.