Dying at age −0.75 counts as nothing, as a little, or counts as a lot of a person, depending on what counts as a person and how much a person at various stages matter. If it counts as a lot of a person, then it leads to an anti-abortion stance, and some pro-abortion arguments might apply in this situation.
And an alternative to abortion is adoption. A person that is highly unlikely to have HD could even be produced on demand by surrogacy or IVF, instead of being taken from the pool of people already up to adoption, so that it is a “net gain”.
If the women would not even consider abortion or surrogacy as better alternatives than giving a high-risk natural birth, I consider that unreasonable and grossly negligent.
There’s a reason very few people label themselvse as pro-abortion in the US. It’s not even a position that’s important enough that gallup asks about it. People who favor the right to abortion in the US generally label as pro-choice.
The general argument based on which abortion is legal in the US is about a woman’s right to her own body being a very important sacred value that trumps the right to life of the fetus.
It’s not that the fetus has no interest in living. It’s rather that his preferences aren’t that valuable given that he hasn’t yet progressed enough in the direction of personhood to overrule the strong sacred value of bodily autonomy.
The US legal system does consider it reasonable to punish people who kill an unborn fetus of a woman without the consent of the woman and doesn’t think the fetus counts for nothing.
Given the cheapness of genetic testing, if you take your own argument seriously the woman in question wouldn’t be the only person with an obligation to stop the pregnancy from resulting in birth.