[Semper]

And, contrary to what you say, they are already in this world, too.

I don't agree. They are totally within the world of a woman's body. They do not breath the air we breath; they do not eat food from this environment; they do not function as we do. They do not see what we see. Everything they experience is through the woman that bears them. Existing in this world means independent functioning and some degree of awareness of this world's environment.

[ClearCase_guy]

If I close my eyes, does the world go away? If I hide behind a door, am I truly invisible? Have I gone out of the world?

A child in it's mother's womb is like a person hiding behind a door. To argue that the child is not in this world is to create a strange, solipsistic world-view.

[The Old Hoosier]

Independent functioning? In that case, no one has any rights until they’re at least 2 years old, maybe 3. And certainly not the old and senile.

It’s a dumb argument. The most vulnerable human beings require the most protection, they can’t just be discarded precisely because they are unable to fend for themselves.