If you are sure about your dates and have regular cycles, when there is a disparity in a sonogram, the best thing to do is to repeat the sonogram, preferably in the trans-vaginal route to see if there is any significant growth in the gestation sac. This may be correlated with serial titres of serum beta-hCG
measurement for a better opinion. Many-a-time, repeat sonogram shows good growth as early scans sometimes are erratic due to poor visualization. Please do not stress; discuss with your obstetrician for further management. Good luck.