Hello all....my positive opk changes each month - can vary by 4 days! Does anyone else have that?. I got a positive today (Wednesday 30th) at noon. Was negative last night at 10pm. We are tracking closely as had 2 BFN so far. We had intercorse Sunday night, Monday night and this morning (wednesday) at 6am. Is it worth trying tonight and tomorrow? Don't fully understand how long your window is...worried we missed the window by not having sex last night.