I know I only did opks for like 2 months before we got pregnant but I feel like before the miscarriage I always got a positive opk by cd 17. Today is cd 19, I think, and still no positive opk. I mean I guess I could have missed it maybe. And I also forgot you aren't suppose to drink much a few hours before you take the test, but still!! Maybe my cycle will get longer or something now after the mc. I heard your cycle can change a lot. I dunno.