I remember reading an article by a catholic philosopher who gave a pretty good argument, but I don't remember the name, so I have listed the argument here.
The argument, fortunately, isn't really that complicated. If the baby is its own organism, alive, and human, then to kill it without just cause would be murder (though, it is never a just cause to kill a baby because it can never do anything to deserve capital punishment inside the womb). The last two requirements that I listed are pretty self-evident. If a thing is made up of cells, then it must by definition be a living thing. If an organism is made up of cells containing human DNA, then from a biological standpoint, it must be human.
The real argument is over whether the baby is its own organism, which can be answered as such:
There are at least two requirements for a thing to be this:
1) The fetus has its own power for self-preservation. The fetus does not need to be told by the mother's brain that it must survive, much like a kidney or a clump of flesh. Rather, the fetus draws upon the mother's nutrients in the same way that we ourselves draw upon the nutrients we find in the refrigerator, completely of our own accord so that we may survive in our environments.
2) It has the ability for self growth. Once the sperm and the egg combine, the baby takes over, and is completely responsible for its own growth. Like they say, you can lead a horse to water, but you can't make it drink. Same for the baby: you give it the nutrients it needs, but the baby is responsible for using those nutrients to make itself grow. No one can make an organism grow except the organism.