Until you have an ultrasound in a few weeks' time, your baby's due date will be calculated by counting 280 days from the first day of your last menstrual period. At the dating scan (see It's really happening!), your baby will be measured and his age calculated. The scan date will then be used since it is considered to be accurate.
While you're bound to want to know the due date, try not to get too fixated on it. Most babies are born within about two weeks of their due dates but your baby will be considered to be born at term if you give birth between 37 and 42 weeks. So your estimated delivery date is just that, an estimate; your baby may be born earlier or later.