Can't gain weight

I see a lot of women worrying about gaining too much weight, but is anyone else having the opposite issue? At first my OB wasn't worried, but now she is starting to kind of press that I really need to gaining weight... and I just can't seem to. You can tell that 'I' am losing weight but that 'baby' is gaining weight and there for 'our' weight is steady. But I'm really starting to worry, I've even added in a carnation instant breakfast drink once a day to try to add extra calories and nutrients.