Actually, most of the numbers I've seen in my researches are in the ballpark of one third to one half, with about one quarter of women being raped before they turn 18. The site I linked to was simply the first that came up in a Google search, so I wouldn't have to dig for references, and so that I could give an estimate on the conservative side.

It's true that such statistics are methodology-sensitive, but everything I know about rape seems to suggest that the weight is heavily toward under-reporting. Women who report being raped are liable to face an onslaught of abuse and victim-blaming from the criminal system and even from their own peers, and rape trials rarely end in conviction, so a lot of victims never bother. Even then, many rape victims suffer from psychological problems (which contribute to their being targeted), and therefore come to believe that they deserved what happened to them, no matter how degrading or violent. In this case they may not conceive of it as rape, especially if the rapist is their partner or spouse.