[supercat]

The joining of sperm and egg is the closest thing we have to an unambiguous event which can be used to distinguish between the period of time during which a given person exists and the time prior to their existence.

In what way is implantation less unambiguous?

[Dr. Octagon]

Because conception, the forming of unique human DNA, happens before it. That's what renders it ambiguous. My DNA, the unique package that determines every physical trait I have, came into existence at conception. Not before, not after. Anything else is a viability test, and there begins the slippery slope.