so far everything is okay. the bleeding isn't coming from the cervix and everything on the…

so far everything is okay. the bleeding isn't coming from the cervix and everything on the ultrasound looked good. the heart beat was actually 116. however, I am confused because the place I had my ultrasound at on Friday, said I was between 5 weeks 6 days - 6 weeks 2 days. but the ER said I am 6 weeks 0 days. I'm so confused ??? are the ultrasounds different? do they measure differently?