Most Accurate Due Date

So I went in for my ultrasound at 8 weeks 5 days and I was told that the baby was measuring 9 weeks 1 day! I've been basing my due date on the first day of my last period as you should, but I'm just curious what the outcome has been for you mamas in the past. I know that babies hardly ever come right on your expected due date, but should I most likely expect to give birth a few days earlier than expected? Or is the baby just growing a little ahead of schedule? Just curious!!