hi there...
i think it depends on who's done the scan.
I just had my first baby by emergency c-section from a failed induction due to size of baby. I had a scan at 32 weeks and 36 weeks to check on size. At 32 weeks my baby was estimated to be 2.5 kgs at 36 weeks- 3.5kgs.
At 38 weeks he was born at 4180- 9lb 3 1/2. so estimates seemed to be largely accurate for me....
mind you, i still dont think i shouldve been induced- but i'm healthy and so is my baby, which is all i can ask for.