With my first bub they changed my dates about 3 times, i had a bleed at what i thought was 8wks they did an internal and said no your only 6 weeks, then when i went for my 12 wk scan based on the internal they said no your 13 wks, etc etc.

As many ppl have said they will generally get you to go off the SCAN dates but i guess you can never be 100%, this time round i am lucky enough that my dates were spot on and they haven't changed me once yet and i am now 20wks.. YAY

good luck with the PG, enjoy it.