Today I was supposed to be 8 weeks pregnant. On Sat. I started spotting, so I went for an Ultrasound this morning. The ultrasound showed a heartbeat and fetal pole, but it said I was only 6w3d. I spoke to my doctor and he said not to worry, that I must have ovulated later that I though. My question is: I always have 28 day cycles, my last period was October 13th (which would make me 8 weeks). What are the chances of the ultrasound being incorrect this early on?

The 8 weeks would be gestational age... counting from your last period. This is partly where the window of time to deliver around your due date comes from because most women are not likely to actually be pregnant right away... often not for a week or two after their period. The ultrasound gives you a closer date to fertilizational age, when you actually conceived.