Congratulations. As for measuring 8days less that is fine. As the measurements at 20weeks is the "average" size at that age there is alot of variation in size for women at that gestation. The closer you are to conception (in the first 2-3months of pregnancy) the more accurate size is due to the fact that time has not allowed bubs to grow long enough to get a great variation between each different baby. The closer you get to your EDD the bigger the variation can be. For example a lady with a growth restricted baby was induced at term (due to growth resitriction) her term U/S was saying she was 31weeks going by the size of her baby on US but they knew from earlier ultrasounds she was term. Bub was born at 1.8kilos. Its the same story for women who have 20weeks scans saying they are a week or two ahead of their original EDD. The correct EDD to go by would be the date of O or date from an earlier scan. Sounds like everything is perfect.