In "Hills Like White Elephants," why does the woman say "It isn't ours anymore"?

Let us focus on what precisely is going on here. We are
privileged observers of a terrifying conversation that is going on between a man and his
partner, who is pregnant with his child. As we carry on listening, we can see that the
man wants Jig, his partner, to have an abortion. She does not want one. He exerts every
kind of psychological pressure on her to try and persuade her and to force her to do
this, including threatening to end the relationship if she doesn't. If she does have
this abortion, however, he promises that everything will return to the way it was before
in their relationship: "We'll be fine afterward. Just like we were
before."

The section you have highlighted comes towards the
end of the story, when it becomes clear that the battle has been one and Jig sees that
there is no way out of this for her. Jig seems to realise that their relationship is
heading for an unhappy finish, and that having an abortion will only speed this process
along. Note what she says and the dialogue she has with her
partner:

"I
said we can have everything."

"We can have
everything."

"No, we
can't."

"We can have the whole
world."

"No, we can't."

"We
can go everywhere."

"No, we can't. It isn't ours any
more."

"It's ours."

"No, it
isn't. And once they take it away, you never get it
back."

Jig seems to be almost
deliberately confusing the baby inside of her with the future hope of their
relationship, as she replies in a series of negative responses to the encouraging and
soothing phrases of her partner.  She seems to recognise the awful finality of abortion
and the way that you can never go back from that, just as in the same way this step will
have massive ramifications for their relationship.