Ultrasound help!

I had a appointment today. They tried to find heart beat but couldn't. So they did a quick ultrasound. Everything was great! Baby was moving all around. However I'm confused because I did my 8 week ultrasound and I ended up being 7 weeks by baby measurement, but this time I'm (11 weeks 2 days) and baby was measuring 12 weeks 1 day which matches up with my original due date. So which due date should I use?