many bitches will loose weight before they start to gain it. Their pregnancy is a lot like humans. Some will have morning sickness while others don't. Humans are pregnant 9 months, dogs 9 weeks...with that in mind, 1 week for a dog is like 1 month for a human. Not very many humans I know that start to show at 3 months (3 weeks for dogs) but will rather start to show in the 6th month (6 weeks for dogs). I wouldn't worry about her size right now.
I'm not a big fan of ultrasounds. While it is cheaper to have one done in my town versus an x-ray, they only saw one pup in Gracie and she ended up having 4.
