As far as life can be said to "begin" anywhen, it begins at conception.
You think womens' rights trump kids' rights or the other way round, okay.
http://lesswrong.com/lw/od/37_ways_that_words_can_be_wrong/ http://wiki.lesswrong.com/wiki/Politics_is_the_Mind-Killer
You're arguing definitions, claiming that your definition of "life" is universal, and using an ambigious definition of "kid" to pull emotional strings. I think we all agree on the anticipated outcomes of a pregnancy. Given how emotional "life" and "kid" are, taboo them.
Can we agree that morality is a set of rules that maximizes global "fun" when executed locally by each person? That's what it is to me. I don't think that it's obvious that the moral definition of "life" is constant, and that we should therefore expect a constant mapping to a biological definition. If you have a morality where allowing all life to end naturally has a constant value that would be quite critical, but I can't think of a society that values all unnatural termination of life equally.
Is it moral to assign more value to one life than to another? Is the life of the head of a household with 7 dependents worth more than the life of a hermit, since his family will take a big "fun" hit?
Do we actually mean ending remaining life, seeing as all lives will end at some point, and you can't take away the ones that already happened? Are some years more fun than others? Are some years in fact negative fun? Do individuals get to decide what is fun for them? Is there some point before which individuals aren't responsible enough to know what will give them the most fun?
Is it less moral to kill someone when they are awake and die in terror than to kill them in their sleep so that their life simply ends and they don't experience any more fun?
Answer a bunch of questions like this and I can determine how immoral it is to terminate a life at fertilization (genetic code is unique except for your identical siblings), gastrulation (no more twins can form), various levels of brain activity (beginnings of a mind), birth (eats, breathes, poops, and communicates), infancy, or cancer-ridden old-age. Arguing over whether something is a "life" or not with no moral context is about as useful as arguing over what a "sound" is.
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.