Your phone can tell you a lot of stuff -- the weather, the news, what your friends' cats are doing at any given moment -- and now it can possibly tell you if you're at a lower risk of getting pregnant today.

Obviously no app is going to prevent STDs, so condoms are still very necessary. But two Swiss physicists, husband and wife Dr. Raoul Scherwitzl and Dr. Elina Berglund, say their new software/hardware combo NaturalCycles is "99.9%" effective at preventing unwanted pregnancy if used correctly, and has led to zero unwanted pregnancies so far out of 10,000 users.

That's a pretty bold claim, and it's based on the science of using a woman's temperature to predict the days of the month when she is at her most fertile. Users start by measuring their temperature "first thing in the morning as soon as possible after waking up," then entering the data into the app. (The creators are working on a thermometer that inputs it automatically.)

Users then follow the fertility readout: If it's a "green" day, then NaturalCycles claims they can't get pregnant; if it's a "red" day, they can. Here's the scientists' reasoning:

"The pre-ovulatory (follicular) phase is estrogen-rich and cooler than the post-ovulary (luteal) phase, where progesterone dominates and ‘warms’ the body. These two phases are separated by the ovulation day...where a temperature rise of 0.2-0.45 °C occurs. The fertile window ends with the ovulation day and begins 5 days before, since sperm can survive 2-5 days in the female body. Only during these ‘red’ days can the woman get pregnant through unprotected sexual intercourse."

It all sounds like very precise measurements and complex science, but the app (which costs $7 to $10 per month) seems pretty straightforward. For women who don't love the birth control pill's side effects, this could make life easier, if it really is 99.9% effective. Just remember, that .1% would make your life a lot less easy, so it's probably smart to have a latex backup plan.