My 13 year old DD had her first period in January. She then has not had another one, or any sign of one coming, since.
She's very slender (5ft 9 tall but size 4-6 clothes) so I suspect it may be because she has quite a low BMI but I'm wondering if I should be taking her to the GP? Older DD started her periods at a similar age and has been pretty regular ever since, so I'm not sure how normal this is!