Calculating How Many Weeks

I am due for my first ultrasound at the end of the month. I know I'm pregnant-- three tests have came out positive and was confirmed by a doctor. I was just wondering how accurate is it to count from the first day of your last period? My last period started Sept. 19th. That would mean I am eight weeks and five days today. Has anyone gone to the doctor and been told they were right or wrong? I am so curious.