'Life begins at conception' is just a nominally secularized reference to the doctrine of ensoulment, not a concept that is morally relevant to question of whether abortion should be legally permissible (or even interesting).

If we cared about the mere fact of life in the relevant way, we'd be interested in legal restrictions on pulling weeds. If the concern was with living tissue with human DNA, we'd be fretting over amputations. Even in the case of human life, not every end to human life is an equivalent of 'murder' to be categorically prohibited to the exclusion of all other considerations. And even if we grant that abortion is always morally wrong, it still does not automatically follow that categorical legal prohibition is a sound social response to the moral issue.

There's an expansive philosophical literature on the ethics of abortion that actually seeks to identify and address the moral core of the issue and its relation to law. The phrase 'life begins at conception' is just a political slogan for a religious dogma that very, very quickly falls apart when you try to use it ss an actual starting point for reasoning about abortion policy. Trotting it out pretty much represents a refusal to engage in good faith with secular thought on the issue.