I also am measuring very small, about 5 1/2 weeks too small. My little girl is also very small, and I have had ultrasounds to track her growth over the entire pregnancy. The last measurement they did on her, she was in the 26th percentile. My pregnancy has been extremely difficult and that is why my baby is so small. I was pregnant with triplets (two identical conjoined babies, and then a singleton). We lost the conjoined babies at about 15 weeks so that is a big reason for our little girl's small size. She had to compete with the conjoined babies for so long and her placenta probably did not develop as well as it should have. As a result, I have really low amniotic fluid. On top of all of this, I had been so extremely sick for the first 6 months (vomitting constantly) and have only gained 7 1/2 pounds.
So, in a nut shell, that is why my little one is small. I would not worry about the size of your baby until you have your ultrasound and know for sure what the size in fact is. I wish you lots of luck and would love to hear how your ultrasound turns out!
Best of luck