I would go by the first dating scan that you had, as the earlier in pregnancy you have the scan, the more accurate it usually is for due dates.
Having said that, dont stress too much about it as most bubs arent born on their due dates anyway.
Aidyn was 8lb7oz at birth (3.8kg) and he was straight into 000's (Bonds) and 0000/NB (Pumpkin Patch & other brands). He grew out of the 0000 before the first two weeks though...
I would doubt that an 8lb baby would fit into 00000, and wouldnt even fit 0000 for very long either.