For the experts on prenancy. What does it mean when there is a conception discrepancy?

My wife is pregnant. We based our childs conception on her last period, which was nine weeks ago. However, after going to our first sonogram the doctor says our child is only about nine weeks along. They said that everything is normal, but that is a pretty big discrepancy in dates. Can this mean there is something wrong with my child? Is it possible that my wife just ovulated later than usual? Should I be worried?

(Please no sarcastic remarks.)