20 weeks pregnant no weight gain
I wouldn't be too concerned. Most of your pregnancy weight is gained in the third trimester. As soon as I found out I was pregnant I dumped all of the bad stuff out of my diet, I gained about 4 pounds during my 1st and 2nd trimester combined. My doc said that I was losing weight from eating better, but gaining baby weight. They sort of cancelled each other out, so nothing was showing on the scale.
Be sure to keep your doctors visits so they have a good record of what's going on during your pregnancy. Your doctor will let you know if any changes need to be made with your diet. Until those changes come, just enjoy this wonderful time :)