There is 15% EITHER side of the *actual* guesstimation that they could be - so babe could be 15% smaller or larger than the size they predict - and that's all it is, a prediction. Did you know that in calculating the babys weight on a scan they use a formula that includes the length of their femur bone? Now call me daft but how does the length of a bone in their leg contribute towards their weight? If you are having a longer than average baby, the estimated weight will be skewed. I had a scan at 39wks with my 4th baby to confirm his position and you could see how chubby he was - he had little boobies and mega chubby cheeks and it was the cutest thing, but my Dr never predicted a size other than that he would possibly be a decent size (he was over 10lb born). I don't see what it matters anyway, you are still going to have a c/s aren't you? (not that size matters for a vaginal birth either)