This happened to me when I was expecting DD1. I knew to the date when I fell pregnant but when I had a scan it gave a later date. The Dr dismissed my date but I stuck to my date and he then said we would work with both dates and see what happened.
She was induced on the date I had and if I had waited longer there was every likely hood she would not have survived.
The scans are only a estimation as every child is different in their growth pattern