We don't even fully understand the concept of consciousness, ie: where does "I" start and where does it end. Asking for an scientific definition of when a fetus becomes self-aware, or even if it deserves to be regarded as a living being before that, is an exercise in futility.
Our concept of life, be it our own or that of animals, are tied to our cultural and religios history. Why is it acceptable to eat chicken but not dogs? simply because we have historically bred chicken as livestocks for food, and it all started as a utilitarian decision, it simply is easier to rear large amount of chickens that it is to rear large amount of dogs.
Going back to the question of defining a fetus's identity, it all boils down to our understanding in the science of cellular division and the cultural definitions we have long adopted as a species (In terms of medical science, this would mostly be the western culture we are talking about).
Well, a philosophical one, maybe. "Let's consider it human when it can form thoughts in its brain", for example.
It started when the first cell ate another cell. Killing for nutrient absorption is an old constant in evolution. We are also social, so we form social circles that include humans and every being that we consider part of ours. It is not about friendship but beyond that, a sense of identity.
But as you say, all of this is mostly conventions, traditions and unchanged beliefs carried from the past into a present when, even if we think about it, we won't be able to change the way humanity as a whole sees anything.