[Bibi]

If you´ve always been slim it will be hard for you to gain weight, at least that´s been my experience. You might want to eat a lot of protein and have a daily protein shake and multivitamin if you start to take Wonderup now.
There are some threads of thin women who´ve grown on herbs (including me) but I think it can take a bit longer and you probably need to concentrate more on having nutritious foods.
hope it helps!