Disclaimer: I am not a doctor all my knowledge comes from Taking Charge of Your Fertility book, FertilityFriend.com and the internet!
But I THINK that you can get a positive and
a) ovulate very late after that (longer than it says on the package)
b) not ovulate at all
This is because the test is detecting the LH hormone which is a signal made in the pituitary gland in your brain to tell your ovaries to release an egg. So you are detecting the signal but you cant detect whether the egg actually gets released (something could go wrong). If all goes according to plan it will be released 12 - 36 after the positive, but everyones bodies are different and indiviudal circumstances like stress can affect the outcome too.
If you want to know that ovulation has definitely happened you can find that out by taking your waking temperature everyday and plotting it on a chart (see fertilityfriend.com)
The best way to use OPK's is in conjunction with other things like checking CM and CP and taking temps, that way you can put all the pieces of the puzzle together and work out whats really going on.
hope that helps fedup
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