What do you favor basing a viability test on? Roe v Wade based it on an arcane survivability/trimester formula that has long since been obsoleted by medical science.
It seems to me the "is it a human yet or not" or "is it a alive yet or not" pro abort arguments are basically dishonest justifications for what they really advocate. It is clear that a fetus is alive, genetically distinct from its mother, and genetically human even at the multi cell stage.
What is truly advocated at heart by both abortion advocates and fetal cell advocates is that the death of one unique human organism is justifiable homicide because of the benefit to another unique human organism.
A society can justify the killing of an attacker, a convicted offender, or an enemy soldier, and absolve the killer of shame and guilt, because such acts are seen to benefit society more than the preservation of the same life. Even so do the abort crowd seek to lay off the cost of their moral choice through a variety of justifications, such as "medical benefit to victims of horrible afflictions" or "mental well-being of the pregnant woman".
In most historically accepted cases of justifiable homicide, the person killed bears some responsibility for the situation. For an unborn human, this is never the case, hence the reluctance to call the killing of the unborn exactly that. By their euphemisms ye shall know them.
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.