Conception Question

I am just trying to figure out when we conceived.... I am 12w4d now. My last period was beginning/mid May, my cycle was very messed up so I don't know when I would have ovulated. When I went to the OB/Gyn July 19th they said the baby measured 6w6d... would that mean that we conceived 6w6d prior to that? Like that sexual encounter was when the sperm made it to the egg? Haha, It's confusing to me because I know some people go from LMP. So I wanted to try to figure out the day/around the day it could have happened.