My view on the matter is that access to abortions falls under the umbrella of the right of bodily autonomy; specifically, protection from being medically exploited. Which by your phraseology would make it a “negative right”.

My go-to comparison is, perhaps oddly, bone marrow donation. Someone with bone cancer is likely doomed to die a horrible death, unless they can find a compatible donor who will consent to share marrow with them. For any given recipient, only a few people at best will be a viable match. Maybe only one. But that person has the absolute right to refuse. You cannot be forced to use your body for the health of another person without your consent.

Some people would say, that’s not comparable to pregnancy, and that getting pregnant is somehow consent. But, at least here in Canada, they stress heavily that you can withdraw consent at any point during the procedure. They also explicitly let you know that, at a certain point in the procedure, the recipient’s bone marrow will have been irradiated, and that if the donor backs out at that point, the recipient will die, but that they’re still allowed to do so. The right to bodily autonomy means any ongoing use of one’s body requires their continued consent, even with a living, breathing human person on the scales. Morally is certainly another question, but the diagram of law and morality is not a perfect circle.

If I’m protected from being the life support of any person, surely that covers an unfinished fetus.