My 12 yo girl is very underweight. how can i help her?

1st-->Physical exam!: First, i share your concern regarding your daughter being underweight. Unfortunately there can be dozens of reasons as to why.... Therefore, the first essential, non-negotiable step is a thorough medical exam by her pediatrician. This most likely will include urine and blood tests, and may possibly include x-rays. The solution to the problem is to clearly identify the cause. Call your doctor now.