Points of balance presume that there's actually something to balance.

The default balance between "I tell you to do stuff" and "you do you" is very far on the "you do you" side, normally. Most of the exceptions are because you doing you actually does impact other people (possibly your future self). Then you try to find a balance.

Abortion is tricky because there's potentially an additional "you" at some point. If one person says, "That's a 'you'!" and the person who is pregnant says, "Nuh-uh", it's exactly the default case again unless there is evidence.

If you want to argue against the individual determination value of Enlightenment liberalism, go for it. But if we even vaguely accept this and/or the reasoning for this, you only get to interfere in "you do you" when you have some evidence. "My imagination pains me, thinking about how you said God's name in vain when nobody was listening" is not generally considered sufficient cause.

So in this case, the entire justification (from a classical liberal perspective) for allowing any interference with abortion is that there is some reality of the situation that demands intervention. Conservative Christians recognize this, which is why they tend to be so adamant that "life begins at conception". They recognize that if the (relevant) life began at some other point, they wouldn't actually have cause to intervene in other people's medical decisions.

This isn't to say that it's easy, or everything is perfectly clear, but if someone wants to argue for 15 weeks, they need to base it in science indicating that a 15-week fetus is, for example, meaningfully a person, and therefore the state has an obligation to protect this person. In the United States, with separation of church and state, "God says so" isn't going to cut it (at 0 weeks, or at 15). Likewise for most other western democracies.