Let my ask a rhetorical question. Why do we need a word "human?" What is the word meant to designate?
Historically the word we use "human" for was "man" as in mankind. The purpose, philosophically, was to differentiate man from all other creatures. The philosophical definition of man is "rational animal." This definition is based on the Porphyrain tree which briefly defines all living things thus: all bodies, non-living (mineral) or living (organism), all organisms, non-sentient (plant) or sentient (animal), all animals, non-rational (brute) or rational (man). So, the ultimate genus (or species) of man is "rational animal." This definition serves to distinguish man (human beings) from all other existents.
To take this very useful concept and use it to make arguments with regard to biological and moral concepts is an epistemological mistake, and does harm to both moral concepts and the basis of knowledge itself, which is the clear non-contadictory definition of words.
My real objection to calling anything other than an organism we can actually see breathing a human being is that it is a confusion of the meaning of the word itself, and all such confusion ultimately allows any kind of obfuscation of the truth reducing all argument to a clamoring of opinion rather than objective reason.
Take the question of abortion for example. This question should never be based on the definition of man or what a human being is. Abortion is the destruction of life, a life that, if born (according to it's strict definition) will be a human being. It is intended to be a human being, and the willful destruction of it will prevent the intended purpose from being fulfilled. [By intention, I do not mean in the teleological sense, but in the biological sense. If you are religious, you may also mean in the teleological sense.]
I believe the argument that an unborn is already a human being does great damage to the "credibility" of those who oppose abortion, encouraging the very thing we object to.