Baby measuring underweight

At our 20 week check up today, the doctor said that our baby is measuring about a week and a half below weight. He said we have to come back in 3 weeks to be sure that she doesn't get behind any further. Has anyone dealt with this? Is there anything I can do to help her grow? Like is a there particular fatty foods that I should be eating? I have lost 2 pounds since I've gotten pregnant, but I haven't been sick at all and I swear I eat constantly!!