There is a hormone called LH (Luteinizing hormone), which is always present in the body in small quantity but surges around the middle of the menstrual cycle when it is released by the pituary gland in a bigger quantity than at any other time in the menstrual cycle. The surge lasts for up to 3 days, and this is the time when a woman is likely to ovulate. Most women ovulate 36 hours after the surge. The surge in LH is evident first in the blood, and then, some 8-12 hours later, it is detected in the urine. Ovulation Predictor tests predict with a 99% accuracy when an LH surge is taking place which is an indicator of a potential ovulation.