at this stage, babies tend to get longer which means they tend to look thinner. Also they do not put on weight as quickly as when they were first born. I have heard that if you give a little bit birds nest each day to your bb, that will help them absorb nutrients and help them put on more weight. But dont worry too much, I am sure your bb is fine. My bb also looked much thinner at that age but then after they reached one, their faces look rounder than before