Ovulation prediction kit, OPK, is a common term among women trying to conceive. It predicts ovulation periods and help determine fertile window. 

As good as OPKs, it has its drawbacks which will lead to miscalculation and wrong timing if not considered. 

Firstly, OPKs can't confirm when ovulation actually occurs. It works with a surge in LH, but there can't be an LH surge without the release of egg. Therefore, OPKs can test positive even when ovulation has not happened. 

Secondly, LH surge is extremely brief. You might test negative in the morning and positive if you test again in the evening. This makes it easy to miss your ovulation periods.

Thirdly, you might experience a false LH surge before the real ones, thereby resulting in wrong timing of intercourse. 

Lastly, OPKs can determine your day of peak fertility and can't alert you a fertility problem. 

So put all these drawbacks in consideration when using OPKs to help your TTC journey.