If you measure your bump when you lie down with a tape measure from the very top to the bottom. It should measure 1cm per week you are.
I'm 37 weeks my doctor check by hand by feeling around my stomach to see how big and the position of the baby.
Some places done do it unless there is a concern. I asked about it too (I'm due Thursday) and they don't do it unless needed
I've only had 1 ultrasound at 20 weeks I don't have another one
Just ask at your next ultrasound. Im very vocal so I had lots of questions. They measure the head, stomach and thighs to give you an estimate. I'll be 38 weeks tomorrow and my little one is abit big weighing 8 lbs 6 oz.