please comment growth ultrasound- 36 weeks

I had my growth ultrasound today.  The doctor said the baby is healthy, but big.  He or she is weighing in at 7lbs 4 oz at 35 weeks 4 days. I need to go for another ultrasound in three weeks to see how big the baby is and if a c section is needed.  So I guess my question is how accurate are the growth ultrasounds? And has anyone been in this situation before? Is this common or is this something to worry about?