Ditching the Pill for an App and an Algorithm

Technically a woman interested in trying the Natural Cycles app is going to need a smartphone and a thermometer, too, as the app depends on the user entering her daily temperature to determine fertility. Peter Ardito/Getty Images

Particle physicist Elina Berglund once worked for CERN on the Large Hadron Collider experiments. But after finding the Higgs boson several years ago with some other folks, she turned to other pursuits. Like, how could a woman track her fertility effectively enough to eschew hormonal birth control?

Berglund developed an app called Natural Cycles that uses a statistical algorithm to determine a woman's fertile days. It relies on a woman taking her temperature every morning; a small spike in temp signals ovulation. Using the temperature data within the larger algorithm, the app predicts what days you should avoid sex if you're not planning a family. (And conversely, what days you should have sex if you are.)

The app has undergone two clinical trials, one of which followed 4,054 women ages 20-35 who used Natural Cycles. They had to pay for access to the app for at least three months and enter their data for at least 20 days. One-hundred forty-three women got pregnant using the app: 10 of those pregnancies were the result of women having sex on days the app had mistakenly dubbed "not fertile"; the rest were women who weren't using the app correctly. That gives a 99.5 percent effective rate. Compare that to the "perfect use" rate of male condoms (98 percent) and IUDs (99.7 percent).

Of course, perfect use isn't realistic. A typical use rate of Natural Cycles is about 93 percent effective, compared to 82 percent for condoms and 91 percent for the pill. And remember: A natural form of birth control won't protect you from sexually transmitted infections.