I wouldn't be too worried. I was told that my bub was average size and he ended up a 9lb whopper! My friend on the other hand was told she was having a HUGE baby (also baby no. 1) and was so scared that she had an elective c-sec. Well bub ended up being a healthy 7lb 11. I don't think they can ever really know and I wouldn't let it put you off going for your VBAC. How big was your first baby, that can often be a good indication?