The central question is when does a being begin being a “person”?
It’s clear to most folks that an individual sperm and an individual egg are not a “person”. It’s also clear to most folks that a child is a “person”. The problem is determining when the status of “person” begins.
It’s important to answer this question because persons are due moral consideration while non-persons may not be.
The argument against abortion is pretty simple…
1) Personhood begins at conception.
2) Abortion stops life processes, i.e. kills a person.
3) Killing is wrong.
4) Therefore, abortion is wrong.
But — all of this relies on the big assumption in #1, namely that being a person begins at conception — and, it’s not so clear that it does.