Hi,

I had my first us today and was told I was 4 weeks. The gestational sac was 8mm.

I did a HPT 21/2 weeks ago and it said I was 2 weeks pregnant (making it 4 weeks at that point). Which would mean I am 61/2 weeks now.

Is it possible they got the date wrong on the scan?

Am pretty worried, so any info would be good!

I'm not sure how the hpt gave the weeks gestation?
The difference is likely gestational age (going from your last period) versus fertilisational age (the time the egg was actually fertilised). This accounts, in part, for the window of time given around the due date for a "normal" delivery time frame.
Scans, while not always exact, are usually closer to the mark... especially early on for dating purposes.