sometimes the scan does the wrong date but usually your period date is accurate. my last period due date was september 3rd but my first scan at 8 weeks said i was due september 19th. I gave birth september 7th and they said she was definently overdue despite being small (6 pounds 15) - apparently you can tell by their skin or something and the placenta? Mind you as the scans progressed it was looking earlier and earlier with the last scan guessing the 10th.