havent had a chance to read the other responces, but always measured behind and Spock was a healthy 3.8kg. I haven't heard the risk of being LBW, I have heard that they tend to be born earlier (another theory of the guess date rather than due date because you know exactly when conception happened with ivf), but Spock was a week and a half over due so who knows =P
hope all goes well with your scan =)