My understanding was that at your 20w scan bub can be 2 weeks either way in size because they're so little anyway....

Based on that theory, I'd say no. I had heaps of 3rd Tri scans with DD and sizing IS NOT ACCURATE. DD was severely growth restricted and had stopped growing and I had a final growth scan 2 days before she was born and it predicted that she would be 2lb 10oz. She popped out 2 days later at 3lb 12oz. I know alot of people who have scans before delivery and there can be 2+ pounds difference in actual birth weight.