I went for my u/s at 14 weeks and the scan said I was a week further along but I wasn't 100% sure when my last AF was, I knew I got it earlier than expected but I still took it from the end of the month, but the dates on the scan said I was in fact a week further along which added up when I worked out that had I got my AF earlier as I knew I had I would be a week further along, so I've gone by the u/s dates and at my 22 week scan the dates still matched up so I'm pretty sure the u/s is correct so I've gone by their dates.