The unmarried teen daughter of Alaska Governor Sarah Palin -- whose pregnancy became a US presidential campaign issue in 2008 -- has reportedly split with her young fiance. Bristol Palin, 18, gave birth to a baby boy in December but has since broken up with the child's father Levi Johnston, 19, People magazine reported. The split happened "a few weeks ago," People said, citing a source close to the couple. "It was a mutual thing," the source said. Bristol Palin in February told Fox News that she planned to marry Johnston after the two finished high school.