Even when the fetus becomes a baby, it still matters. You are forcing another person to help that baby, in a much stronger (albeit temporary) way than any other.

Think about it; you are not required to help somebody if they're in an accident or a fire; you're not required to donate blood, but you would require a woman to donate blood and nutrients, spent several months of hardship and have a small but non-zero risk of serious health problems.

At some point it changes; in my mind, around viability. If the baby is delivered, you cannot kill it (you may be required to take care of it, although many states allow you to give up your kids).

Somewhere around 6 months, we might require a woman to give birth... Maybe