Actually a baby's brain takes about 1 year to become self-aware, of course they have brain activity just like non-sentient animals have. Although it would be barbaric to kill a baby that is already born, so birth seems like as good a line as any to defining a conscious human being, even if they aren't sentient as of yet.
The fact is we are an impossibly lucky group, out of the uncountable trillions of potential DNA combinations we have been blessed with life. It seems strange to claim killing a zygote is any more cruel than eliminating the chance for that zygote to ever be born. It is true we are killing something that has the possibility to become a sentient human, but again possibilities should not dictate our action as that would lead us back to sperm.
An argument could be made that the brain of a fetus experiences pain during a late-term abortion, although rudimentary nerve cells in an extremely premature fetus probably also experience some level of pain reception. In both cases the fetus is not self-aware, which is how we justify our cruelty unto animals that experience pain.
However, that all said, I would be disappointed if someone I knew wanted to undergo an abortion. It would be truly sad to lose the possibility for sentient life, but I dont think we can legislate a womans uterus which harbors a parasitical mammal that is not yet self-aware, despite the impossibly beautiful thing it might be capable of.