Conception date

So iam 3 Weeks my dr said yesterday as per blood test and iam only going for an ultrasound in 2 weeks time when i would be around 5 weeks.So iam trying to figure out when about conception happened so i can work out the due date ,Can you girls help me get a rough idea of when.Here are the dates we Baby Danced i haven’t had a period since going off BC in Nov so i have no idea when i ovulated.