how long have you been measuring behind? who is doing the measuring?
So by the book- Varney would say if you have had 2 measures that are more than 2 cm different than weeks refer for ultrasound
how has your baby's movement been? is your baby low in the pelvis or already enguaged? if you have always measured different than dates then it would be less of a concern to me, maybe just how you grow um -- so you have charts? because there can be a bit of holding on the sperm/egg meeting .