I was measured at 26cm at 29 weeks and all was fine - the ultrasound confirmed that baby was in the 40th percentile. The sonographer mentioned that she didn't know why people with my stature (slim) were sent in for u/s when it was obvious that I was just a small build. When I saw the ob the following week, he said pretty much the same thing. It's also to do with where the baby is as well, so try not to worry, although I know it's hard!