40 women with no known infertility issues were followed during an entire menstrual cycle. Each morning they collected first morning urine, which was used to measure estrogen (in the form of E3G), FSH, LH, and Progesterone (in the form of PdG). Ovulation was tracked and confirmed by ultrasound. They discovered that many of the LH surges in the urine of these "normal" women were occurring at the time of ovulation or AFTER. They conclude that measuring LH surge is not a reliable method of tracking ovulation before it starts in many women! 

Possible Reasons Why an LH Surge is Detected AFTER Ovulation?

 1)  An LH surge in the blood causes ovulation, therefore, by the time it is detected in urine, ovulation could have taken place.

 2)  An LH surge is quick (can be as short as 12 hours). This causes some women to miss the surge or only catch the tail end, thus missing ovulation.

So, next time you measure your LH surge, keep this in mind. If you're trying to conceive, your most fertile days might be the days BEFORE the positive LH test, not the 24-36 hours AFTER your positive LH test. 

Information was taken from 2015 article by Sarah Johnson et. al. "Development of the first urinary reproductive  hormone ranges referenced to independently determined  ovulation day." Complete article can be found here.