Weight gain with twins!

Ally • We have a beautiful 5 year old teenager 😂. And expecting boy/girl twins in April 2016!
Hello I just have a question... I am 16 weeks with twins and weighted myself this morning and I haven't gained not even a pound. My belly is already noticeable and I can feel the babies move once in a while. But my weight concerns me a bit. When did you start to gain weight?🤔