As you say, the doctor will give you a closer answer regarding your due date.
Do you know when your last period was? Then you can determine your due date.
Still, which ever method of figuring out your due date you and your practitioner use, remember that due dates are merely guesses. Most babies are born two weeks before or two weeks after your due date.
I recommend you use this tool to get an approximate date: http://www.babycenter.com.au/tools/duedate/