Answer: Hello dear. Your due date is calculated by adding 280 days (40 weeks) to the first day of your last menstrual period (28 day cycle). Otherwise your first pregnancy scan will probably happen when you're between 6 - 8 weeks pregnant. It's known as the dating scan, and is used to give you a more accurate due date, based on the size and development of your baby.
Unfortunately, no one can say for sure exactly when your baby will arrive. In fact, only about 4% of babies are born on their due date. But it is likely that your baby will arrive in the few weeks either side of your estimated due date. Knowing this can help you to be prepared for labour and birth, and to have everything ready for your new baby.