Looking at it purely scientifically, here's the fact of the matter:

Before conception, you just have a sperm and an egg, and at the moment of conception, you still have a sperm and an egg. The material doesn't change, but the positioning of it does. The blastocyte (a collection of stem cells that forms the foundation of the human body) forms a couple of days after conception. Before then, the sperm could exit the egg, leaving you right where you started.

This fact is contrary to one key characteristic of humans: You cannot revert back to the sperm and egg you came from. At some point after conception, the process of fertilization becomes irreversible. The sperm and the egg merge into one cell and cannot be separated without fundamentally altering the material at hand. The scientific argument for when human life begins may be made no earlier than this point.