Medical abortion is no different than shooting an adult in the head, when you get down to it.
To be a bit more accurate, it's more like being a politician that orders some third world guerrilla leader to be assassinated. They don't shoot the guerrilla leader in the head themselves, but in the chain of people that made it happen, they bear the most ethical responsibility for the action. It's a bit easier being the person that gives the orders, less messy and easier to rationalize away. Human beings are big fans of such things.
Often people will debate abortion on theological grounds. In the United States, that's often the only grounds that abortion is debated on. Theology is problematic though, as your debating opponent has to be of the same religion or close enough that the theological grounds your argument rest on hold sound in their view of the world. In other words, an evangelical christian is going to have a hard time convincing an atheist that abortion is wrong. Mostly because the desires of some god the atheist doesn't even believe in tend not to hold weight for them in any argument.
And if the two people in an argument about abortion are of two different religions that have differing views on abortion? Phew! Talk about your lost causes.
Reason, however, is universal to all humans. People might not agree on the existence of a supreme being, or even one that's particular on how people get killed, but every human being with two brain cells to rub together can understand why the statement '
sheep eat grass, therefore all things that eat grass are sheep' happens to be problematic.
Most human beings will say that shooting people in the head isn't a good thing in general. Human life tends to be a universal good in every human society, given how you'll find prohibitions and laws against murder in pretty much every one of them. It *is* easier to kill someone than not if you disagree with them in a heated argument. Especially if you both have guns. Take disputes about cheating in gambling in the Wild West era for example.
But if you go around shooting everyone you have a problem with, society just goes down the drain. Getting good dental care will be tough if the dentist thinks you'll have no problem shooting him in the head when you see the bill. This applies to every facet of society in general - murder isn't something that can be tolerated if you want civilization to work well.
Murdering people who are classified as being outside of a given society is far less problematic though. People from another country that become enemies, people of a different race, color or creed, etc. Gradually people have come to understand that even with these differences, people are no less human for them. The life of a Civil Rights protester is worth no less than that of any other person's life, as it turns out.
"Clearly it must be so Socrates!"
If we can all agree through logic that human life and the preservation of such is a good that we all value, the next question is 'where does human life begin?'
This is often the topic of fierce debate - is a human being still human if it's only a few cells? If it's so many months old in the womb? Only after it has drawn its first breath of air? Only after it has the right to vote?
No one argues against the idea that a fetus *will* be human at some point. To shed light upon this dilemma, perhaps it makes more sense to ask at what stage something will not be human. In other words, if you grow a kidney in a laboratory and keep it alive, it isn't going to become a human being in the natural course of events. A sperm by itself, will never become a human being. Nor will an ovum.
However, get the two together and unless the natural course of events is interrupted, the result will end up a human being. Perhaps even one that ends up reading this, who can say?
Is a human being any less a human being at any stage in their life? Is the life of an elderly person of any less worth than that of one in the prime of their life? Logic implies that human life, regardless of the stage it is in, is worth equally as much at every point from beginning to end.
While it might be hard to declare when something is a human being in the world of biological science, in the world of reason, logic and philosophy, it is far less so. Logic implies that it is human unless it is not. While that statement may seem simplistic, it is very important. For it is much easier to decide what will never and can never be a human being than what is a human being.
Given the two groups, things that will never and can never be human beings, and things that are not present in the first group; eg a baby, one can safely say that killing things in the first group could never be considered murder in the sense of one human killing another.
That leaves that second group then. Things you can kill that are or will be human. Logically, if a squalling infant turns into a polite adult with table manners, the squalling infant is no less human for its lack of table manners. Continuing in that chain of logic, a zygote is just as human as an individual of voting age is.
Logic implies that killing a zygote is an act of no less weight than shooting someone in the head.