The differences between people who argue for and against abortion is so extreme that the two sides cannot be reconciled. The avenues of their arguments are rarely logical, but rather political with catchy yet meaningless phrases. The two sides are digging their trenches deeper and deeper: deadlock, in other words. In the most extreme case, an atheist with an acknowledged selfishness cannot reconcile with an ultra-conservative Christian who is damning him to a Hell that he does not believe in. However, where politics cannot change, logic supercedes. The only two real differences between pro-choicers and pro-lifers is the definition of a human and the parasitic qualities of (or lack thereof) a fetus/unborn baby. Only these two differences exist because abortion is only wrong (and also illegal) if determined that the fetus is a human and that this human is not a parasitic stowaway hitching an unwelcome ride. The pro-lifer must prove that the fetus is human and that this human is not parasitic.
[By the process of elimination, we will see that the only time that a human becomes human is at conception.]
Some people, such as members of numerous Protestant denominations, believe that abortions can only happen before the 3rd trimester (6-9 months). The logic for this is quite valid: the 6-month old is developed enough to look and feel like a human baby. In some cases, it can even survive outside the womb, although quite underdeveloped. Under this guise though, a fetus in the 2nd trimester (3-5 months) is less developed and thus less worthy of life than a more developed 3rd trimester fetus. However, biological processes such as abortions are not clear-cut, "line in the sand" examples. Just as variances in real life exist about development (14 year-olds males with beards versus their non-bearded 16 year-old counterparts), abortion is no different. The developmental cycle in any biological being (whether it be a bacterium or a fetus or a massive whale) is not statistically exact. Contrary to the lack of imprecision of the stock market (which can be pinpointed between quarters, weeks, days, and even minutes), the trimester of a pregnancy is rather an inexact arbitration with real variances. A 5-month 3-week old fetus (still in the 2nd trimester) can look more advanced than a 6-month 1-week old fetus (already in the 3rd trimester) because embryonic development is not fixed in stone and can vary like any other biological process, as with the 14-year old bearded male. (No wonder most fetuses aren't born exactly on their 9-month due date.)
The same argument can be applied against the logic by which humanness is defined at birth. Why can a 9-month old fetus be aborted (though rarely, but still legally) a day before delivery while a premature 8-month old fetus that has been delivered can be allowed to live? Believing that the potentially underdeveloped 8-month old fetus, or say 7-month old fetus, can live just because it is on the "outside" while the "ready" 9-month old fetus can be aborted is against common sense.
There obviously is no defining feature of humanness either during the pregnancy cycle or birth itself and because of this the "born, human, alive" argument is dead. Since the same developmental argument can be applied to any later stage (for example, why an already born baby isn't a human yet), process of elimination tells us one only one thing: either we are never human or we are human at conception, the only feasible biological beginning of the individual. As a result, the only logical conclusion is that we are humans from the first moment of conception.
We can also look at genetics for defining what it is that makes us human. At conception, the sperm (with its own genetic material) and the egg (with yet another set of genetic material) create an embryo with distinct genetic material. This new genetic material is capable to, and usually does, turn into the same human being that created it. So, if that embryo will become similar to its two creators, either it must be a human like them, or none of the three (man, woman, embryo) is human. Taking it to its logical conclusion, if no one is human, then I could shoot anybody who, for example, disagreed with me since there is no human life that has to be respected.
[Having proved that the fetus is a human and cannot be anything other than a human, I have to prove that this human is not a parasite of the mother. When that is proved, abortion no longer is plausible since we are in effect killing a harmless human.]
Reasonable outcome tells us that the human hitching a ride inside the mother is not parasitic. As opposed to a cancerous tumor, the final result of the unborn human is to be similar to its parents, and thus helps reproduce the human race, the same race the mother is from. A cancer patient will die because the cancer will literally take life away from its host. Cancer is truly a parasite: if left untreated it will, in almost all cases, cause permanent death. On the other hand, the unborn human is taking a very limited supply of the mother, as the mother can perform all vital functions needed for maintaining her own life. This "sapping" without regard for what the "woman can actually spare" is not grave, vital, or threatening to the mother's life or even health. The discomfort, as opposed to a feasible threat, of pregnancy is temporary, showing that the unborn human is not malignant. While there are a few tragic situations when the mother does die as a result of the pregnancy, it cannot be generalized just as cases in which cancer patients who mysteriously survive cannot be generalized either. (As a small sidenote, if the mother is in imminent threat and danger from dying as a result of the birth, the mother's life is to be preferred since she has proved herself to be more capable of life than the unborn child.) Having already proved that the fetus is a human, the "comforts over life" argument is illogical. For example, if I get stuck working with a boss who causes me a pay cut or makes me stay an hour after regular hours, then because of this discomfort, I could legally kill him. As people feuding over their grievances of personal discomfort would cause anarchy (as you could kill anyone who causes you discomfort), the temporary discomfort of pregnancy is to be of lesser consideration than the new human life's value.
I have proved that a fetus is a human and that this unborn human is not a parasite. All other arguments are squished underneath because now abortion deems as killing of a human life. No amounts of "fetus enslaving mother" can shatter the inherent evil of abortion because no amounts of temporary discomfort, if even that, can justify the killing of a human life.
[While pro-choicers might be left in the cold by this essay, let the fact that someone who read this essay thought that I was for abortions calm your nerves.]