Jig's statement about that which has passed encompasses so much about the relationship she shares with the American and her own life. The point in the story in which she says this is the point where she has brought out her own hesitation towards "the procedure" and where the American has shown himself to be the one advancing Jig to have it. At the same time, Jig has already indicated that she "does not care about" her own state of being. The exchange that follows between them is a reflection of the supposedly positive ideas of the American (actually seeking to simplify and reduce the complexity of the situation) and her own despair. Jig's assertion that freedom and the ability to "have the whole world" in the shared dreams with the American is impossible because there has been some barrier crossed, some frontier in their relationship passed, a demarcation where all has changed. It is at this moment in the dialogue between them where some level of change has happened. The optimism and the freedom with which both the American and Jig travelled the world over, as seen with the stickers on their suitcases, and the sense of self- absorption in which both lived for and with one another has become replaced with a sense of uncertainty and a lack of clarity. This vision of the future, where what will be is pushed into the realm of insecurity, is something that motivates Jig to speak out of a position of despair, and in stark contrast to the American.
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."
"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.
With "It isn't ours anymore" the woman is referring to the world.
Because the couple have been corrupted and broken by the deed that they know they must do, their innocence has been taken away forever.
That can never not know how things were - they cannot see the world as an innocent, beautiful place. They now can only see it is a place of darkness and loss. The world was once theirs, bright and new, but now they are changed and can never go back to the way it once was.