Last year, a small Swedish startup made waves with what it called the world's first form of "digital contraception." The company's product, a smartphone app called Natural Cycles, pairs with a thermometer to track women's basal temperature every day, then uses that data to make predictions about ovulation. Rather than curbing ovulation, like an oral contraceptive, Natural Cycles gives women either a red light or a green light on unprotected sex depending on when they're most likely to be ovulating. The app promised a 21st-century update to contraception—one that used algorithms, not hormones; one that lived on an iPhone, not inside of a woman's body.

That promise is now under investigation, after a hospital in Stockholm reported last week that 37 out of 668 women seeking abortions since September had used Natural Cycles as their primary form of contraception.

That's just one hospital, in one city. The app reportedly counts over half a million subscribers across 160 countries. Chances are, more than just a few dozen women in Stockholm have been failed by the app and others like it.

The report from Stockholm is interesting because last year, Natural Cycles became the first app to be certified as a contraceptive in Europe. It raised millions of dollars in investments during a moment when interest in consumer health technology is staggeringly high. So the fact that women are reporting unwanted pregnancies from Natural Cycles already, not even a year after its certification, is not just alarming. It's a miner's canary for a much larger constellation of contraceptive technology.

The Algorithm Method

Before there was Natural Cycles, there was Clue, Ovia, Kindara, and dozens of other apps for charting one's fertility. Some of these apps look like digital calendars of menstruation: They provide a space on a woman's smartphone to log periods and track cycles over time. Others use period tracking, as well as data like basal temperature, to predict ovulation and suggest windows of peak fertility (for women trying to get pregnant) or low fertility (for women trying to avoid pregnancy).

"All these apps are really souped-up rhythm methods." Reproductive clinician Mary Jane Minkin, also known as Madame Ovary.

It's true that ovulation is cyclical, and tracking data over time can help a woman predict when she's most likely to conceive. At best, apps like Natural Cycles give women space to log their own bodily rhythms and understand when they're most likely to get pregnant. At worst, they take folkloric advice about how to not get pregnant and make it seem more credible by dressing it up as a smartphone app.

"All these apps are really souped-up rhythm methods," says Mary Jane Minkin, a practicing gynecologist and reproductive clinician at the Yale University School of Medicine. "The term for the technique was known for years as 'Vatican roulette.' And the old joke was: 'What do you call women who use the rhythm method? Mothers.'"

Even still, it's not hard to find reasons why women would find a cycle-tracking app appealing. The burdens of contraception are high, and fall largely on women. Hormonal options can wreak havoc on the body, causing all kinds of unpleasant side effects. Without insurance, birth control pills are expensive, and often out of reach for young or low-income women. IUDs can be painful, condoms can be uncomfortable, emergency contraception can be fallible. So it's forgivable that a natural method—something that requires little more than monitoring your own body and downloading an app—seems appealing. Consider the group of women New York Magazine once called "the pull-out generation"—young females fed up with hormonal birth control and interested in understanding their bodies more deeply. Those women gave rise to an ecosystem of apps that claimed to hold all the information—and not just information, but technology, right there on your smartphone—needed to master one's own body.