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Per PEOPLE:

Bristol Palin and her fiancé Levi Johnston have broken up, two sources tell PEOPLE.

The split happened "a few weeks ago," according to a source close to the couple, but it's unclear what precipitated it. "It was a mutual thing," adds the source.

Bristol, the 18-year-old daughter of Alaska Governor Sarah Palin, spoke with FOX News in February and told Greta Van Susteren that she and Levi – who are parents to 2-month-old son, Tripp – expected to get married after they completed high school.

"It kind of just happened," says the source, referring to the split. "I thought they would stick it out. But I think they can work together to raise Tripp."

"I'm not sure what caused [them to break up] – it's common knowledge," says another source who knows the family.

Despite the breakup, Levi still sees the couple's son. Levi's dad, Keith Johnston, told PEOPLE recently that his son is a devoted and "proud father."

Plans for the Future
Bristol, meanwhile, is attending Wasilla High, taking a class to supplement course work she is completing at home. She also is considering enrolling in college next fall and studying nursing.

Bristol has been spotted around Wasilla shopping and exercising. She recently returned from Juneau where she was visiting her mother, who is spending the winter in the state's capital with daughter Piper and son Trig during the legislative session.

As for how she is holding up after the split, the source tells PEOPLE: "Bristol's doing okay. Tripp is fine."

Representatives for Gov. Palin have not returned calls for comment.