My daughter tracked along the 3rd percentile the whole pregnancy. I had to have growth scans at the end. OB was never too concerned because she started small and continued to track small (along 3rd centile) and was born at 40 weeks weighing 2.67 kg. Completely healthy and thriving. She's now 3 and still petite but developmentally strong. My understanding is that the concerns are if the growth rate drops off rather than continuing along a lower but consistent percentile. I'm now 21 weeks with my second pregnancy and also a small but healthy baby. Apparently my sister was the same and she's now tall. Good luck.