According to Associate Professor Maureen Condic: it begins at conception. Her white paper, "When Does Life Begin?", is online from the Westchester Institute. Here's an excerpt of an interview she gave Zenit.org (emphasis mine):
Q: You define the moment of conception as the second it takes for the sperm and egg to fuse and form a zygote. What were the scientific principles you used to arrive at this conclusion?
Condic: The central question of "when does human life begin" can be stated in a somewhat different way: When do sperm and egg cease to be, and what kind of thing takes their place once they cease to be?
To address this question scientifically, we need to rely on sound scientific argument and on the factual evidence. Scientists make distinctions between different cell types (for example, sperm, egg and the cell they produce at fertilization) based on two simple criteria: Cells are known to be different because they are made of different components and because they behave in distinct ways.
These two criteria are used throughout the scientific enterprise to distinguish one cell type from another, and they are the basis of all scientific (as opposed to arbitrary, faith-based or political) distinctions. I have applied these two criteria to the scientific data concerning fertilization, and they are the basis for the conclusion that a new human organism comes into existence at the moment of sperm-egg fusion.