due date by ultrasound

I am due on February 8 th, I had irregular periods so we had a ultrasound to date our pregnancy . Us dated 6 weeks 6 days -dating back til may 4 th .. 
I'm trying to figure out when I conceived, dr said we conceived sometime in may most likely around may 17th or 18th .. 
Anyone else in the same boat trying to figure out when they conceived ?