Some thoughts that I don't remember anyone expressing on LW.
First let's get this out of the way: life does not "begin at birth". As far as life can be said to "begin" anywhen, it begins at conception. Moreover, the child's intellectual abilities, self-awareness or similar qualities don't undergo any abrupt change at birth. It's just an arbitrary moment in the child's development. So it would seem that allowing killing kids only before they're born is illogical. What are the odds that your threshold for "personhood" coincides so well with the moment of birth? Could it be okay to kill kids up to 2 years old, say? CronoDAS voices this opinion here.
But there's another argument in favor of considering the moment of birth "special". Eliezer linked to a study showing that the degree of parental grief over a child's death, when plotted against the child's age, follows the same curve as the child's reproductive potential plotted against age. Now, the reproductive potential of an unborn kid depends on its chance of survival, and the moment of birth is special in this respect. In the ancestral environment many kids used to die at birth. And mothers died often too, which made their kids less likely to survive. An unborn kid is a creature that hasn't yet passed this big and sharply defined hurdle, so we instinctively discount our sympathy for its reproductive potential by a large factor without knowing why.
How much this should influence our modern attitudes toward abortion, if at all, is another question entirely. As medicine becomes better, kids and mothers become more likely to survive. So if our attitudes were allowed to drift toward a new evolutionary equilibrium which took account of technology, we'd come to hate abortions again (thx Morendil). But then again, the new evolutionary equilibrium is probably a very nasty system of values that no one in their right mind would embrace now (won't spell it out, use your imagination).
Ultimately your morality is up to you and the little voices in your head. You think womens' rights trump kids' rights or the other way round, okay. But if you use factual arguments, try to make sure they are correct.
ETA: see DanArmak's and Sniffnoy's comments for simpler explanations. Taken together, they sound more convincing to me than my own idea.