My first pregnancy, I had all these so-called professionals telling me bump, baby etc was too small - and there is no way to weigh a foetus. They can estimate mass, but it isn't a sure thing. I had that too. Trying to scare and bully me for months. But I was more confident than you, by the sounds of things.

DS was big when born.

Is baby kicking yet? If so, that's the best check, not tape measures and scans. And healthy mamas with kicking babies very rarely have prem babies. Yes, some do, but it is very unlikely.