Couples with fertility problems can often improve their chances of conceiving by accurately predicting when and if the woman is ovulating. Available in more than a dozen different forms, ovulation predictors are making it easier for women to detect when they are ovulating. With so many varieties on the market, it can be difficult to know which one is right for you.

"The trigger that causes ovulation to occur is the increased release of luteinizing hormone (LH) from the pituitary gland," says Dr. Randy Morris, a reproductive endocrinologist from Naperville, Ill. "Before ovulation, the LH levels are low. At some point, the LH levels begin to rise rapidly. This is called the LH surge. The duration of time that the LH levels are high is about 48 hours from start to finish. Release of the egg from the ovary happens about 35 to 44 hours after the onset of the surge."

When comparing urine to saliva ovulation testers, it is important to understand how each works, and why.

The LH in the blood is eventually filtered through the kidney, which causes the levels in the urine to increase, Dr. Morris says. "When a level of LH is present in the urine, it causes an indicator to change colors for most testers," he says. "A 'control' indicator is usually given to compare the change in color. Ovulation usually occurs in 12 to 24 hours."

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