They work by measuring the LH in your urine, which is released a day or two before you ovulate, so it lets you know when you're going to ovulate. You pee on the stick, preferably at the same time every day, and I believe late morning/early afternoon is best, and when the test line is as dark as or darker than the control line, you have a positive and you should ovulate within a day or two. They can be kind-of hard to interpret sometimes, unless you get the digital ones that give you a smiley face for a positive (close to $30 for 7-10 days' worth of tests, as opposed to about $10 for 7-10 days of the lines, or about $20 for 20 days, usually cheapest at Wal-Mart). They just tell you your most fertile time, at which point there's still only a 25-33% chance of getting pregnant, so they're not fool-proof, but if used correctly they can help maximize your chances.
at 12:45 PM on Mar. 1, 2009