Conceiving just before AF

So my husband and I Bd 2 days before AF. We are ttc and as of today I am 3 days late on AF. I have tested several times and they are All a bfn . Could I have to wait (and if so for how long after BD ) to test for me to have enough hgc in my body to register a positive . I saw somewhere that you should test 20 days after you bd if you don't know you AF time. Any help?