Every living organism on earth has a DNA sequence, which varies by species. Tiger DNA is not the same as Giraffe DNA. There are some species that can make sterile hybrid offspring (Ligers, Tigons, Mules) due to similarity in the DNA, but the DNA of each parent species is not an exact match.
Therefore, there exists a DNA sequence that is uniquely human.
When determining whether a particular cell, or cells, are alive, we can observe one or more of the following phenomenon -- mitosis (cellular division where the DNA is duplicated), cellular respiration, as a result of osmosis or photosynthesis, etc. We can clearly observe and define life in these parameters.
At the time of conception, shortly after the sperm cell penetrates the egg cell, we can observe mitosis and cellular respiration. It is clear that at a biological level, this new organism is unquestionably alive.
This new organism also has a complete human DNA sequence, which means it is unquestionably, biologically, a human life.
Because of these Scientific Facts (in fact, you could even say "The Science is Settled") Human Life begins at conception, with each new person having the potential to develop into an adult organism.
The abortophile will often question this with, "Well, what about miscarriages? See, even God performs abortions so it should be legal!"
This argument does not hold up to a logical analysis. Sometimes, the process of human reproduction fails and the result is a miscarriage, or premature still birth. Some times, there is no explanation for a in-utero death. The reality is, people of all ages die every day of natural causes. Applied to it's logical conclusion, this argument can also be used to say murder should be legal, because all people are going to die anyway. Clearly, this is incorrect, and as a result, using miscarriages to defend abortion falls flat.
Referring to the unborn child as "pregnancy tissue" or "fetus" or "not really a baby" is also deliberately misleading by abortophiles. My second son was born too early, and died as a result. There was no question at all, just by looking at him, anyone could tell that this was unquestionably a tiny little boy.
The next argument made by pro-abortionists is often the fact that the unborn child is dependent on the mother for survival; that it is just a parasite. This is another weak argument, as no one disputes that a tapeworm is alive. While an unborn child does meet the biological definition of a parasite, this does not mean the unborn child is not alive, nor does it mean the child is not human. Furthermore, the average 6 month old is also dependent on the mother for care, and this is not unique amongst humans. Young being dependent on parents is a common trait amongst mammals and some other groups as well - birds, for example, care for their young. Furthermore, some Democrats consider people to be dependent children up to age 26.
Therefore, the only logical conclusion is that abortion is the ending of a human life through non-natural means. It is killing a person (and in the case of the mother, wounding her as well). This ending of an innocent life can not be justified in cases of rape or incest, as the child had no control over it's creation, and does not deserve a death sentence for being conceived under horrible circumstances.
From a medical ethics perspective, the only time an abortion can be justified is when the life of the mother is at risk. The reality of this situation is that BOTH lives are at risk, and the goal is to save the lives you can â" in this case, the mother's life -- because if the mother's life ends, the child's life does as well. Any other case -- there is no moral, medical, ethical, or any other reason to justify abortion. It is not a "medical procedure", it is a crime that when carried out successfully leaves 1 human life dead and the mother wounded.