Question about my conception date

Okay so here’s the skinny, my husband and I tried for two years and back in August ‘17 found out we were finally Preggers. I specifically remember having sex on July 4th, thought maybe I had a period on the 8th which lasted about as long as usual with maybe a period of clotting and it was done on the 11thish which is about the length of my periods anyways. Then I read up on implantation bleeding so I thought maybe that’s what it was. When I went for my first ultrasound I was predicted to be 9w 3d but Baby was measuring 8w 2d which would have put the date around the 16th. Was that most likely a period I had on the 8th? Of course we have sex all the time so it was more than likely a period but I just want to know the accuracy of ultrasound dates as I couldn’t imagine she’d be that off in the early ultrasounds