More formidible minds than mine ought to address this and expound on your thoughful notion, but here's my 'first thought'.
The adult human is well adapted to living in the environment around us (outside the womb). The form and function of the complexity that is an individual human being in the adult stage are thus the defining characteristics for that age. To ask 'when do these form and function characteristics cease to define the being' has been addressed by societies, and now medical science, ever since, I suppose, we began dealing with our dead. At present, medical science uses a 'death protocol' when contemplating organ harvesting from an individual body. But it would be useful to note that the definition for 'dead' is also based on the form and function that previously defined 'alive'. That form and function notion, if applied logically to the emryo, would argue for a being present in utero because the embryo is well adapted in form and function for life at that age in the continuum. There isn't really any absurdity in such an apprroach and it would argue for protecting the individual human life present in embryonic age/stage.