I am kind of a trimester, continuum kind of guy, with parental notification mandatory except for a court order where patental abuse is proven. I focus on the sentience of the fetus. It is not for everybody, but I think that is where the vital center is on this issue. And we need to clean out the detritus of court activism on this issue, so that it can be discussed and voted upon in the public square. JMO.
Sentience is not a good marker, since the human species has demonstrated sentience as a general rule, so have cetaceans and some non-human primates. On the other hand, it is not permissible to act to purposefully and electively kill non-sentient humans who have been born (at least in most of the world). I wouldn't demonstrate sentience under general anesthesia, but that wouldn't make me a candidate for purposeful killiing, would it?
Viability is not a good marker, either, as has been recognized by the courts. Viability is an ever-changing marker, one which moves to earlier and earlier stages of development. Humans who are not viable this year might be next year, and more than likely would be in years to come.
Science has no problem identifying human life or that of any other species. Lab technicians and their supervising physicians decide which in vitro fertilization embryos are to be implanted and which are not. The purpose of somatic cell nuclear transfer would be " is precisely to produce just such an entity: one that is alive (rather than nonliving), one that is human (rather than nonhuman or animal), and one that is an embryo, an entity capable of developing into an articulated organismic whole (rather than just a somatic cell capable only of replication into more of the same cell type). "" (The President's Bioethics Council, Report on Human Cloning, Chapter Three:On Terminology
http://www.bioethics.gov/reports/cloningreport/terminology.html And just try to justify the destruction of the egg of a bird on the Endangered Species list by claiming that the egg was simply an embryo or fetus and not viable or sentient - you'll find yourself guilty of breaking Federal Law.
Let's stick to scientific, taxonomical definition of humans rather than personal beliefs and biases that serve only to discriminate and infringe on the inalienable rights of some human beings at the whim of other, more powerful human beings.