jules I went through this. I was measuring over 2 weeks smaller than i should be and had to go for a late scan. I wad increadibly worried just like you and ds is now a healthy 9 month old As has been said scans can be a pound out. Ds was born 7lbs12oz btw, not huge but absolutly no concerns Enjoy the rest of your pregnancy- and those cream cakes
I went for a 3-D scan at around 33 or 34 weeks I think and they said, oh, you've got a small baby in there. I had to go for another scan at 39 weeks as the midwife couldn't find the head and they said oh, you've got a big baby in there (with head still firmly attached). DD was 7 lbs at 10 days overdue.
I had the same thing with dd (now 3yrs old). They kept saying I was measuring small and did a couple of late scans which seemed to indicate that she was "small but healthy". Like you I was worried sick about this at the time but dd ended up being absolutely fine. 11 days late and 6lb 6oz.
I am due on Sunday with second dc and this time they keep saying this one isn't a tiddler, and I am measuring average etc. Of course now I am worrying that this baby is going to be huge!!