Hi there! I am a 16/18, and I also haven't gained any weight. I too lost some in my first trimester, but have been steady ever since. My Dr. is fine with it, and says that baby is just fine. YOU actually may be loosing weight, while baby is gaining. I don't know about you, but I am eating much differently than I was before I got pregnant. My appetite is reduced so my portion sizes are much smaller, and I am not eating (or craving) nearly as many sweets as I was before. I have also gone off of diet soda (I will have ONE regular soda once or twice a week). So, for me, I think I am likely just eating better.