"Not much point in going to a healer for that, though," Ron replied, taking the tea from Hermione and holding the warm mug in his hands, "It'll be a boy."

With six in his family and Hermione being an only child, it only made sense. Maybe Bill and Fleur had a girl, but that was only because Fleur had a sister, wasn't it? You didn't need a Muggle healer to work it out.