The site author, Scott F. Gilbert (author of the textbook Developmental Biology) has this to say about the question of when human life begins:
The "answer" is fluid, in that it has been changing throughout history, because any answer about when human life begins is deeply integrated with the beliefs, values and social constructs of the community or individual that drew the conclusion. Throughout history there have been several "answers" to the question of when human life begins, but the only consistency among the answers is that they are always changing as social contexts change, religious morals fluctuate, or new knowledge about the process of embryo development is obtained.Gilbert's account of how human life has been viewed throughout history is really interesting, but if you scroll down, you can see where he describes the "Current Scientific Views of When Human Life Begins". I don't need to tell you that each of these views comes with its own political implications that influence the contemporary controversy over reproductive choice. Defining the point at which human life begins is inherently political, whether it is said to be at fertilization (the most widely held religious view), gastrulation/implantation in the uterus (the official position of the British government), first neural activity, the development of the nervous system (and therefore the ability to feel sensations like pain), or the development of working lungs (which allow a fetus to survive outside the womb).
For an even more detailed description of these different views, I recommend reading The Facts of Life: Science and the Abortion Controversy by Harold J. Morowitz and James Trefil. It will certainly make you think twice about accepting it as fact anytime someone asserts that life begins at conception. ("Conception", by the way, is an event that doctors have absolutely no way of determining that it has taken place. Pregnancy cannot be detected until the zygote implants in the uterus, and it is predicted that at least half of all zygotes abort naturally before they ever make it there. Scientists say fertilization may not even be an "event" at all, but a fluid process that lasts up to 22 hours.)