"Hills Like White Elephants" is a short story where the dialogue and the words are extremely important. The plot/situation is revealed through the dialogue. The tone and personality of the characters is also revealed through word choice and dialogue.
When Jig, the girl, concludes the conversation by saying she is "fine," she is trying to end the conversation because it is useless. If she keeps the baby, she supposes she will lose the American. If she aborts the baby, she supposes she could keep the American. However, either scenario is a guess. Since she can't be sure and wants her disingenuous partner to drop the subject, she says she is fine, but of course she is not fine. She is very unsure of how either decision will play out.
She uses the word "fine" to end the conversation but also because of what it has meant within the context of their conversation. In attempts to put her mind at ease, but also subtly push for the abortion, the American says how they will be after the abortion takes place:
We'll be fine afterward. Just like we were before."
Earlier, the American tells Jig, "Let's just try and have a fine time." For the American, "fine" means fun, no responsibilities: the carefree life they had been having.
Being that Jig uses "fine" to conclude the conversation and that the American uses "fine" to describe a state of being without a baby, the reader might conclude that this means Jig is conceding and agreeing to have the abortion. But this is a tenuous argument because Jig is more intent on ending the conversation than making a direct decision. In the end, her decision is unclear. She uses "fine" to shut him up because she knows "fine" for him refers to that carefree lifestyle.