Help with EDD

MrsJRob

I am trying to figure my EDD! I mc'd in July and never got AF and low and behold I am pg in August! yippeee!! I didn't get my BFP in July till I was 4w5d (POAS almost daily until positive). Since I didn't know what to expect after not getting AF, I POAS and BFP, shocking! I then went in for an elective u/s and the tech, who can't give you medical advice, estimated I was 5 days earlier than I thought. However, that would mean I got my BFP at 3w6d?!

My question is since I didn't get a positive until 4w5d(the 1st time), does that mean I have to at least be close to that to ever get positive? I assumed I just produce lower amounts of hcg then the test can read until at least 4w+? I have a u/s with my Dr. who will give me a real EDD  in 10 days, but that's far away...