How do I know how far gone I am??

Hello,
After a year and a half of trying we found out this weekend that I am finally pregnant ( just as the doctor was recommending us to the fatility clinic)
I don't have regular periods I can be anywhere from 20 days to my longest at 58 days?? 
It has now been 63 days since my last period, but I have no idea when I ovulated how can I work out how far gone I am? Also will my doctor just make me wait 8 weeks for a scan to make sure it's 12 weeks or can I get one earlier???
Has anyone else had this? And what did you do? And what did your doctor do?? 
Thank you