[supercat]

Because conception, the forming of unique human DNA, happens before it. That's what renders it ambiguous.

In this context, for something to be ambiguous means that it's difficult or impossible to tell if it's happened. Implantation is not ambiguous; either it has happened or it hasn't.

[Dr. Octagon]

Conception--joining of sperm and egg, resulting in combined DNA---happens, or not. Perceptibility does not adversely affect the existence of a particular action, circumstance, or result.

Implantation is ambiguous not because of doubt regarding it's advent, but rather because it is the second stage. It comes after the above-described joining---conception---at which the blueprint for a human's entire development is established. Prenatal development, and postnatal development.

The establishment of DNA here noted does not exist prior to conception, nor does it change after.

Anything else is a viability argument.