Obviously there is someone to blame here, but I think it could be possible to miss this for a couple of reasons.
One being that her parents might have had no reason to expect that she had her periods yet, so nothing to miss. The other being that she may already have been overweight.
Additionally, a child being pregnant may not "present" a very normal pregnancy anyway - possibly a small child.
Which is not intended to defend any negligence in any way, but there's no point in a witch hunt on such scant facts when the family may well be innocent.