Well, first, I'm not sure that "viability" is the proper inquiry. There is too often imbedded in viability arguments the notion of survivability independent of the mother.
I tend to view uterine implantation as the demarcation point. Of course, post-implantation there remain a large number of natural fetal development failures, but it seems to me that post-implantation there should be no artificial inducements of failure. In short, I think the very definition of the term abortion is artificial termination post-implantation.
There are, obviously, problems with even this apparently bright line. In the implantation process, enzymes in the trophoblast of the blastocyst effectively break down the uterine lining, and this erosion of both the superficial epithelium and the deeper, cellular connective tissue is a process that takes several days. It is really only after the blastocyst is completely buried that cell differentiation commences within the inner cell mass. So you have a period of time between commencement of implantation and successful implantation during which there is arguably no inititation of fetal development, and in fact a period of time during which natural failure rates are fairly high.
That said, there is a definable moment when the implantation process commences (even if it is difficult to ascertain) and hence a definable bar to artificial de-implantation processes, including those that would interrupt the implantation process once commenced.
Indeed. One could logically point out that a three year old could likely not survive without a nurturer providing food, shelter and protection from harm.
What is your opinion of medical experimentation that artificially sustains and grows the blastocyst and induces later stages of development that normally occur after implantation without implantation? It is a fairly straightforward problem to keep the blastocyst developing in a laboratory setting, getting differentiation and such, without the tissue every being implanted in a womb.
Certainly it is reasonable to assume that before too many years pass, medical science will permit us to grow fetuses all the way to the point of birth, or decanting, as Aldous Huxley called it. Should such a child have rights as an individual, even though it was never implanted?