Not yet having become a father, I am not one to place a whole lot of stock in as far as parenting issues and things like this go. However, I do want to throw my thoughts into this discussion real quick, and then I'll jump back out and see what happens.
From my perspective, a man and a woman are one flesh; that is a special union ordained by God. Lo, children are an heritage from the Lord; however, they do not intrude (or should not!) upon the bond between the man and his wife. As someone once said, children are but guests in their parents' house for 18-20-something years. Guests that should be taken care of, loved, and protected at all costs, but not when that comes between the parents and their unique, God-ordained union. My parents loved me, but they loved each other first, and that is how it's supposed to be.
I know that for women this can be hard; especially for mothers or expectant mothers. While killing a child is never a good thing for any reason, sometimes there is little choice when it comes to the life of the mother. Granted these times are extremely rare, and the chances of that occurring to anyone involved in this particular discussion are very slim, but the principle should be established.
Something else: some say that things of this nature are a test of faith; while this may or may not be true, saying that someone, in this hypothetical situation, lacks faith because he or she decides to take the life of the child, is rather retarded. Locking your door at night isn't a lack of faith in God: it's common sense.
Overall, I think it's important to step back and try to view the situation from others' perspectives, be it that of someone else in this discussion, someone of the opposite gender, or a completely uninvolved third party. Things tend to clear themselves up once you get a wider angled view of the situation.