The wise folks at Stand to Reason have a great website with a ton insight on faith in the public square. They focus on topics like bioethics and philosophy. They came up with the SLED test, which Shane loves to share every chance he gets. I'll see if I can do a decent job summarizing it-but you can always go to the original source.
This argument shows why abortion can't be justified, simply using logic, not scripture (many people don't care what scripture says):
S-Size: My husband is larger than me, that doesn't make him more of a person. Ash weighs about 60 lbs more than her 20 month old sister, but they both have value, neither has more than the other. An unborn child is much smaller than a toddler, but as illustrated above, value isn't assigned to people according to size.
L-Level of Development-Having a crop of kids give me great examples for this. Obviously, my oldest child has a higher skill levels than our toddler. That doesn't make the toddler less of a person, it just means the toddler must still mature and grow. A baby in the womb is less developed than a 6 month old, but level of development doesn't change personhood.
E-Environment-I am the same person if I am at work or at home. The same person if I'm on a plane or stepping off a plane. Environment does not change personhood. A baby's environment-either in the womb or just arrived from it, doesn't change its value.
D-Degree of Dependency. Many people try to say that because a baby is dependent on its mother, that's reason enough allow abortion. Are we not dependent on anything? I'm dependent on the electric company to bring me power, to Wal-Mart to offer the food I don't grow myself, I depend on my employer for a paycheck. Dependency doesn't make us not human or without value. Dependency is not a valid reason to allow abortion.
So, even though the mother may not have planned this pregnancy, we can't offer abortion as an option because the unborn does have value, worth, and deserves the same protection as all other humans.
The SLED test assumes that life begins at conception, but there are several other schools of thought out there, one even states that life/personhood begins when a child is much older, I believe 9 months old. I'll blog about this soon.