If the newborn is suddenly stopped from suckling on the tit, it DOESN'T die. Not at all the same thing.
I'm not quite sure what you're trying to prove. I've already admitted that the dependence of a foetus on its mother is in some ways unique - mostly in the sense that the foetus is dependent on one person whereas a newborn baby can in theory substitute another person for the source of its nutrients, warmth, shelter, etc.
Still, I'm really not sure where this gets you. I'm still not persuaded by the suggestion that a foetus isn't a person just because it is wholly dependent upon another person quite simply because I think everyone is dependent on other people. The fact that a foetus is dependent (at least immediately) on one person whereas I am dependent on hundreds of them doesn't seem to me to create a qualitative difference between the foetus and me. It places additional weighty responsibilities on the mother, to be sure, but it does not - to me - mean that the foetus is simply part of her any more than I am part of my employer just because I rely on him for my wages, or my milkman because I rely on him for something to put on my cornflakes.