There's no one answer to your question, because this is a live debate, and different thinkers have very different moral intuitions about it.  *Orthodox Catholic theology* represents perhaps the position most to the end of one extreme, that **the fetus has full personhood** from the moment of conception.

At the other end of the spectrum, *utilitarian philosopher Peter Singer* is perhaps the visible figure who has most fully embraced your line of reasoning with all its uncomfortable ramifications --in his point of view, a young child, an intelligent animal, and mentally disabled adult should all be viewed as **essentially equivalent** based on their intellectual capacity and ability to live a rich life.  He would argue that if it is justified to kill any one of those, it would be justified to kill any of the others given the necessary equivalences.