I don't trust Dr and ultrasound guesstimations at all and think it is nothing but a PITA that ends up worrying the mums too much.
I was told that I was measuring fine with my eldest DS and not once was anything suggested that I be induced... He was born a week late and weighed 9lb7oz.
However, with my DD and DS2 I was constantly told I was measuring large for dates, and the babies were large on ultrasounds, at one point with my DS2 a midwife voiced her concern for me carrying such a large baby as I am only 5'3... and CS was mentioned for my DD as they guessed her as being close to 11lbs. DD was born 7lb9oz and DS2 was born 7lb10oz and absolutely way off what was estimated. It annoyed the cr@p out of me because when being told I might need a CS with DD, I broke down in tears and worried for the rest of my pregnancy.
PLEASE don't let them tell you that you will need something when it is most likely not necessary.