There are four major differences between a fetus and a grown person, which can be demonstrated with the acronym S-L-E-D:
1) Size- Is this a factor in human-ness? Is Shaquille O'Neal more human than Vern Troyer (Mini-Me)? If not, then a fetus can not be considered less human than an adult due to it's size.
2) Level of Intelligence- Is this a factor of human-ness? Is Bill Gates more human than Jessica Simpson? (Seriously) If not, then a fetus cannot be considered any less human due to its lower level of intelligence.
3) Environment- Is this a factor of human-ness? Do you become more human when you walk outdoors and less human when you return inside? If not, then a fetus cannot be considered any less human due to its consistant position within the mother.
4) Dependancy- Is this a factor in human-ness? Is a child less human than an adult because he/she cannot survive on their own (cook, provide)? If not, then a fetus cannot be considered less human due to its physical dependancy on a mother.
The truth is, an embryo is completely developed in terms of its DNA before pregnancy is even recognized. There is nothing to be added to the basic chemical structure after this, only time to grow. Chemically, a fetus is identical to an infant, a child, an adult, or an elderly person. The only way to deny the human-ness of an embryo is to deny the metaphysical nature of it, namely to say it has no soul. This will be awfully hard to prove for the god-less Planned Parenthood. Do you see this catch-22? Either we are only material beings (according to materialists and secular progressives) so therefore an embryo is as human as you or I, or we are also spiritual beings in which we must rely on a higher source of wisdom to tell us what constitutes human-ness and when it begins. Therefore, unless a diety has told us otherwise, we should view embryos as our equals and stop the legalized mass-murder that is abortion on demand.