[bandie91]

> in the same way that a skin cell is a human cell but is not itself a human.

IMO, there is a substantial difference between a conceived egg and any other cells: an egg turns into a human being over time (provided it's left doing its businnes normally), other kind of cells don't operate this way AFAIK.

> there is some point during pregnancy where an instance of a human exists where one did not previously

completely agree. IMO we can even extend it by omitting the "If a single cell is not an instance of a human" condition: even if we qualify a specific single cell to be a human, there must be a point when it became a human.