I violinist has hopes and dreams, definite social relationships, opinions, values, an awareness of her own mortality, moral autonomy, &c.; a fetus has none of these. I don't think it has to be much more complicated than that, unless you want to submit journal articles for analytic philosophy, in which case you would of course be much more precise with your terms.
Babies don't have these either, but it's hard to draw a line; more to the point, a baby doesn't compromise anyone's bodily autonomy.