Wish mine were too Weirdly, this bubby was conceived in my shortest cycle ever since monitoring it, thanks to the naturopath! (I think it was about 33 days, they were usually 37-45, sucks don't it ) I'd still be BD-ing every second day if you've got fertile CM, although you can get patches of it before O. You could still do one just in case it is a freaky cycle, but probably hold onto them until at least CD18 to start testing, maybe CD20 in case you have a short luteal cycle. One of the best things about charting and using opks is that you really get to know if there are strange things in your cycle that might be stopping you conceiving (short luteal, anovulation etc). Helps with evidence if you ever need to see someone.