That seems a bit off to me. Is this your first baby...i'm guessing no since its a scheduled c-section. The most I ever measured ahead was a week or so. With my second I actually measured a week behind. I was lucky to have a doctor that did an ultrasound at every appointment, thats just his policy.
Are you certain when you last period was? Another question would be if your significant other is of large stature. We have a friend who had their baby 6 weeks early and he weighed 6 lb 8 oz (only 4 oz less than our 39 weeker). Her husband is big and they said their baby would weigh around 10 lbs at birth if full term. At 34 weeks she was rather huge too.
The ultrasound should be fairly close to the correct size. At 38 weeks our girl was measuring about 7 1/2 lbs and she was born at 6 lb 12 oz. So it is off a bit.
Good luck with your little one!