It's not likely to get a positive just 6 days after ovulation is it?

I am sure that I conceived on July 6th due to the following facts, my last two sono's have me measuring a due date of March 29th which would make me 38 weeks 4 days, plus I got my bfp on the 17th of July. According to the doctor and my first sono (which the lady didn't know what she was doing) I conceived on the 11th of July making my due date April 3rd and only 37 weeks 6 days. But that would be impossible wouldn't it? How could I possibly get a positive test just 6 days past conception? Your input is welcome