I'm having around 31 day cycle and I'm of age 30 . I have been on letrozole 2.5mg in my last cycle( first time) CD 3-7 . My OB suggested to have intercourse on CD 13-17, but when I checked with OPK , it showed negative from CD 13-20 . could anyone tell me if I could have ovulated earlier than CD 13 or is there any chance that I could ovulate anyday from CD 21-24 or so . Could anyone please help me with this.