Hi Rose

I had an US at 35.5 weeks and they est my bubs weight to be 3.08kg (6p 13oz), then the doctor said that she could gain another pound or so before birth. When she was born (at 39 weeks + 1 day) her weight was 3.38kg (7p 7oz) So gained 300g in the last 4 weeks or so. So going by that, the scan was probably pretty well spot on.
But I have heard of them being quite out with estimations.

All the best