A “monitory of zero” clearly has no need to be protected by a legal system.

Just like we don’t legislate that you can’t kill plants for the sake of the plant, a fetus spends up to 18 weeks in a non-sentient state where it has no subjective experience. It doesn’t need protection since a non-sentient being can’t suffer.

Edit: it’s also worth mentioning that many abortions happen at the zygote or embryo stage, before there is even a fetus.