The physical state of the fetus is not in question; the 'surprising discovery' here would be that an abortion has some quality of badness, one which is not implied by a subjective observer's desires or a full and complete understanding of the physical system.

I think I have two responses:

• Firstly, I sometimes am convinced to change my mind on moral issues as a result of purely moral arguments. Something like moral uncertainty seems to be at play.
• I think there's some danger of equivocation with "full and complete understanding of the physical system." Maybe if I knew the position of each atom, and had all the systems-level understanding that would imply, then there would be no moral uncertainty. But it seems possible that I could have a 'full understanding' in the conventional, more banal sense, and also have moral uncertainty, even if some strong version of physicalism is true.