As for "premature" births, there really isn't a set time when all babies come out. Sure most come out after 9-10 months of gestation, but some more and some less. Naturally shortened gestation periods have adverse effects on most babies (low birth weight, etc) but once that baby is born and breathes on its own it is "independently life sustaining" no matter how long it was gestating. By your logic any woman who has had a still born is guilty of murder or at least involuntary manslaughter.
Rights are tricky thing; the human mind tends to be very self-centered in regards to them. There are many rights that I do not use for one reason or another. For instance I have never owned a gun (outside of the military) and have no interest in becoming a gun owner. If Congress were to repeal the Second Amendment, many people who are of like mind would shrug their shoulders and go about their day because the loss of that right doesn't directly effect them. I, on the other hand would be right up there with the NRA dorks taking up arms and revolting. I don't want the Government taking any of my rights away from me. Telling a woman that she must carry this dependent organism until it is alive just because you don't like it for whatever reason (usually religious but not always) is taking away her right to control her own body. You don't own anyone's uteris, neither do I nor the US Government nor even a zygote.
Besides, just because something is outlawed doesn't mean it stops. Pot is illegal, in my state even for medicinal purposes, but I know where to buy it and it's easy to do so. Harder than it would be if I could go to my local 7-Eleven and pick up an eighth of Blueberry Yum Yum, and riskier because one usually deals with folks who are not of the best character, but I can still get it. If Roe v. Wade were somehow overturned all you would see is more women dying from "back alley" abortions. Oh yeah, and those precious zygotes that everyone is worried about would be gone too.
Abortion is, to my definition, and the definition of several others, murder. Plain and simple. There's a being, a zygote, a fetus, whatever, that is being fed, that is growing, and that is a human being. A doctor takes care of the fetus and the mother, not just the mother, so to my definition, there's a life in the womb.
However, I understand you and several others also do not consider an unborn child a life, as much as I may disagree with that. You are also right in saying that making abortion illegal will not stop people from having back-alley abortions, though the drug analogy does not work in this case. I find heroin abuse deplorable, but I think it should be legal because a heroin abuser is only abusing oneself, it's that person's choice. An abortion, if defined as murder of a life, is a mother murdering her unborn child, who has no choice.
The problem is there is a large portion of society who considers abortion murder and a large portion of society who does not. So which is it in a legal sense? I am comfortable, on a personal level, with my definition of abortion, but you are also right in saying making abortion illegal will not stop abortions and increase back alley abortions just like the drug wars do not stop drug use and promotes violent drug gangs.
This is a really difficult issue for me in a legal sense, if not a moral one. It's funny, because Libertarians generally agree on most issues, but the one issue where I see dissent amongst Libertarians is abortion. Right now I lean on the side of abortion is murder, murder is wrong because it's force on another and therefore should not be permitted in society, but at the same time, I'm admittedly more shakeable in a legal sense on this issue than I am most others. It's something to think about, it's something I wrestle with.