These are very tough issues that society struggles with. For that reason, there is often inconsistencies between laws.
But the bottom line is that only a pregnant woman has the legal right to abort (and thus kill) the fetus/child she is carrying.
A person who murders a pregnant woman had to right to kill her and/or to kill her fetus/child.
I am also very familiar with the concept of viable life. I believe that a fetus is alive from the moment of conception. But I also believe that all humans have the right to self defense and to their own body. For a woman this includes the right to determine if she is willing to carry a baby to term.
Years ago I had a terrible pregnancy that ended with the still birth of twins late in the 6th month. During this pregnancy I lost 25 lbs. I could not keep any food or water down. If I sat up in bed, I was so dizzy that I would throw up. I had to be hospitalized several times just to get re-hydrated. Were it not for blood transfusions and heavy antibiotics I would have died from that birth. I was disabled by infections for 6 months after the birth. Note that I have only very lightly touched on the physical and health problems caused by that pregnancy. Just don't think you all want to be bored by the long list of hell.
In the end, the infections following that pregnancy made it so that I could never get pregnant again. And it took years for my health to recover from that pregnancy.
In the old days, 25% of women died from child birth. Today women still die from child birth but at a lower rate.
After I started to get better, my then husband wanted me to get pregnant again immediately. I told him that I would only agree to get pregnant again if I had the choice to an abortion should the next pregnancy get to the point that the previous one did. He reluctantly agreed.
See, my doctor never even suggested that I might want to consider abortion because the pregnancy was taking such a toll on my health. My doctor did not do that because he had zero idea of the real hell I went through in that pregnancy. Sometimes only the pregnant woman knows the real toll that a pregnancy is taking on her. So she needs to have the right to make the decision based on things that only she knows.
That pregnancy is the reason that I support choice. That pregnancy taught me that a woman is the only one who really knows what is going on with her body and mind. And like every other human on this planet, that woman has the right to self defense.
Based on my real-life experience, one that I only learned because I went through the pregnancy from hell, nothing anyone says against choice will convince me that a woman should not have the right to protect her mental and physical health during pregnancy--to the point of her having the right to abort.
In natural law, every human has the right to self-defense.
Do some people use abortion as birth control? I'm sure that some do. But in the end, that is between them and God.
The self defense argument is really the only argument that's ever swayed me on this. I do oppose abortion, not on religious grounds but on ethical. I believe that a "being" with the requisite set of 24 chromosomes constitutes a separate human life form, regardless of developmental status. Arguments of ensoulment do not enter into my thinking on this. I believe that every "being" (human) has a fundamental right to exist. I oppose the death penalty as well.
My thoughts on ending the life of another human being are that it's basically only ok in terms of self defense.
Every child a wanted child? That would be nice, but here's my response: I'm not a wanted man or husband--I'm not giving up my right to exist without a fight. A child that's not wanted will likely not have a pleasant childhood, but that doesn't take away from his/her right to exist. Emotional or substantial support from a parental figure does not magically imbue a being's life with the right to exist.
Cases of rape/incest (I count most cases of incest as rape since they normally involve someone underage): pregnancy resulting from this is regrettable (horrible) however since the being in question possesses the requisite 24 chromosomes, they are not only a separate life form with the right to exist, they are also innocent of any crime that resulted in their creation. Execute the rapist, not the child if an execution is what is demanded. The pregnancy and birth the mother must endure is part of the crime inflicted upon her, but the life of the child is not.
Which leaves me with the self defense argument. I agree with Elegirl that every human being possesses the right to self defense against any outside life that threatens it. I think if a 15 year old man-child takes a swing at you, you should be able to knock him on his ass in self defense without fear of punishment under the law. If a fetus (although I still believe it to be a separate human being) threatens your life, it is a threat, and I believe you should have the choice to defend yourself.