my experience with them hasnt been positive.
with DD2 i had a growth scan 2 days before she was born and they told me she was behind for dates.
she was born 2 days later (4 weeks early) and was 7pd 8 (3550gms) and was 50cms long. i fail to see how that is behind for dates!!!
i wouldnt worry to much about a big bubba. the only time they can tell how much they weigh is when theyre actually here!
Good luck