There is also implantation (on the uterine wall) and viability. Viability is fairly vague but implantation is not. If you say life begins at fertilization then you must be against both the pill and IUDs (which is fine) not all pro life people would agree; which brings me back to my original point, the real question is when does human life begin?
The 'discussion' so far is bordering on the ghoulish, as different posters try to haggle over the worthiness of these individual human beings, seeking some definitive age of the individual before which the society will count these individual human lives worthy of protection.
Before this gets much further, let it be said that if American society does not get this right, now, at this juncture in our national life, we will be embracing cannibalism soon as if it is enlightened medical application for 'cures' for the older in our society by harvesting the body parts of the younger (stem cells are the body parts of the individual human being at their embryonic age along their individual continuum of life in their body). Does our society value individual human life?
Individual human life begins at conception. There is no point following fecundation where an individual life may be halted that doesn't terminate a human being's existence. Let me reiterate that one. Halting the continuum of individual life begun at conception ends an individual human life already in existence. Will we 'bottom out' at cannibalizing the earliest age of the individual? No, that will still not be the ultimate denegration of humanity, but it will signal our end collectively. The slippery slope has one last abomination : the harvesting of identical twin individuals supported for a few weeks or months until their tissues develop to the stage 'most desired for harvesting to treat the older twin'.
It must stop now, or we will have no recourse to cease the abomination.
I'm definitely against IUDs. I have a friend who managed to be born because his mother didn't have hers pulled (thus aborting him) when she became pregnant. That's abortion. The Pill is a little more ambiguous (because there are a few ways it could work) but, yes, it makes me uncomfortable. There are more than enough blocking forms of contraceptives out there (including condoms, sponges, diaphrams, and sterilization) that I would rather err on the side of safety and have neither the Pill or the IUD. Note that my opinion is not based on how "convenient" it is or what it does or doesn't include.
As for implantation or viability, neither is really a criteria for "personhood", although they can be raised with respect to the woman's responsibility. If you apply either implantation or viability criteria to an animal or alien, you aren't going to be able to use it to determine who is a person and who isn't.
Bear in mind that no human infant is truly "viable" without care. Exposure was a common form of infanticide for a reason. Exposing an infant to the elements, whether you do it before birth through abortion or after birth by leaving it on a rock illustrates how artificial this difference is. Yes, before birth, the mother is the only person that can care for the child but that doesn't change anything. If you and an infant were the only survivors of a plane crash on a remote island, I think it would be your responsibility to take care of the infant even though you didn't want it, you don't have to take care of it, and it may be a pain in the neck. As for implantation, purposely not catching a baby dropped from a burning building, if you are the only person who could catch and save it, is a problem, wouldn't you agree? If you tried to catch it and failed, I wouldn't hold you responsible. But if you purposely opened up your arms and said, "I'm not catching it, that's something else entirely."