Mandy, I'm not a doctor, but a constant reader. As I understand ketones in urin are showing that your total amount of carbohidrates are not enought, so the body use your saved fat. This is good for a diet that could burns fat, but never for a pregnant woman.
ketonic bodies are found on people that is starving or in a really low carbo diet.
Please, be calm but try to incorporate as soon as you can sweet fruits (bananas, apples, peaches), and other meals. Good example are potatotes and bread. While more liquid you drink, less posibility to retain glucose (carbos)
While I was pregnant of my daughter I didnt eat nothing that a soup during a whole day. I lost so much weight and my doctor just said "good for you, your lossing weight". I lost the normal level of water and get bad. A new doctor sent me to a good nutrition.
My daughter is so thin. I dont know if it was because of that.
Small, frequent meals will be best for you. Its hard when you are pregnant, but shoot for 6 small meals a day. Each meal should contain some protein, some fats, and some carbohydrate. Just a little will be fine.
Ketones have more to do with not eating enough than with not drinking enough. 6 eight ounce glass of water should be plenty per day. Do not drink so much that you feel full all the time.
Good luck with your scan, keep us posted.