How much weight you should gain depends on how much you weigh to begin with and if it is a single fetus or multiple. In a single pregnancy, if you are underweight (BMI 30) to begin with, goal weight gain is 11-20 pounds. These weight gains are for the entirety of the pregnancy, not at any certain point in time. Certainly, as the fetus gets bigger, you will gain more weight than at the beginning. However, to be at 26 weeks and not have gained any weight seems unusual. You should see your OB and make sure everything is progressing as it should be.
All pregnant women are different. At 26 weeks your Dr would probably like you to have gained a few pounds.But, this may not start happening for awhile. Are you having trouble eating? Are you taking prenatal vitamins?