Claire~ Welcome!
A positive opk is when the test line is AS DARK as the control line. Sometimes it is difficult to catch your lh surge, but is usully easiest if you use an opk 2 times a day. For me I used afternoon and evening. But, just because you do not get a positive opk, does not mean you will not ovulate.

As far as bd... my RE told me to bd every other day over the time you suspect ovulation. If you are charting, it will only take you a couple of cycles to figure out when that will be. Most people are fairly consistent with their o day, within a day or so.

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