If your in the UK and have a growth chart in your notes you should be able to see where the growth may end up at 40weeks.
I done at 36 and my baby weight is 2.6 kg
They do it at 34 or 35 weeks, I got mine at 34 weeks
When I'm turning 34 weeks today
You should have a fetal growth ultrasound done in order to find out about the weight of the baby.