The Food and Drug Administration announced Friday it was giving its official approval to Natural Cycles, an app that claims to be able to help prevent pregnancy. It’s a historic move, the first time the FDA is allowing an app to market itself as a contraceptive. The government agency is also establishing criteria and guidelines that will pave the way for a new category of apps that can be used as digital contraception.

Natural Cycles is part of the burgeoning sector of “femtech” — technology aimed at women’s health, which analysts estimate will become a $50 billion market by 2025. Period trackers are one of the fastest-growing categories, with apps like Clue, Glow, Period Tracker, Kindara, and Eve offering services like period calendars and push notifications for taking medicine.

While these apps track menstrual cycles and calculate ovulation windows, none of them have been given the type of approval that Natural Cycles has. The app also received classification as a medical device in Europe by the inspection and certification company Tüv Süd in 2017.

Natural Cycles is at the center of a heated conversation about the rules of contraception and what should be promoted to young women. Lauren Streicher, a professor of clinical obstetrics and gynecology at Northwestern University’s Feinberg School of Medicine, said an app like Natural Cycles is “problematic on so many levels.” She said the FDA’s approval of the technology left her infuriated and speechless.

“This isn’t science; this is craziness,” Streicher said. “We’ve already developed good, safe, reliable methods of contraception that are available to us. This app is completely taking women back in time.”

How Natural Cycles works

The app was created in Stockholm by husband-and-wife duo Elina Berglund and Raoul Scherwitzl. It uses an algorithm that tracks body temperature and period cycle information and is designed to determine which days users can have sex without getting pregnant. For $79.99 a year, users are sent a basal thermometer, although users can also pay $9.99 a month and use their own. (A basal thermometer is more sensitive than other thermometers, according to the FDA, and can pick up on the slight rise of a woman’s body temperature during ovulation.)

Once a user’s temperature information is plugged into the app, users get feedback: Green indicates the user is not ovulating and therefore does not need contraception, while red means the user is ovulating and should not have unprotected sex. The app tells users that it takes between one and three cycles for its recommendations to be consistent.

This method, known as fertility awareness, is not new. Calculating ovulation windows based on body temperature and menstrual cycle has been done for decades, especially among women who hold religious or ideological values and don’t necessarily subscribe to the idea of birth control pills or condoms. The Catholic Church, for example, promotes family planning for couples who want to manage when they conceive children.

This method has also gained followers in chic hippie communities (Goop, obviously, has written about fertility awareness). A 1995 book, Taking Charge of Your Fertility, was written by women’s health educator Toni Weschler, who promotes this fertility method for health and wellness reasons (and is not religiously motivated); her book is currently the best-seller in Amazon’s fertility section.

Natural Cycles has more than 900,000 users, and co-founder Scherwitzl has said that the company’s goal is for the app to get certified as a medical device in every country.

”Consumers are increasingly using digital health technologies to inform their everyday health decisions, and this new app can provide an effective method of contraception if it’s used carefully and correctly,” Terri Cornelison, the FDA’s assistant director for the health of women at its Center for Devices and Radiological Health, noted in a press release.

Medical experts say the science behind the app isn’t always effective

Natural Cycle’s reliance on a basal thermometer sets users up for failure because it requires exact use as part of a routine, according to Streicher.

“We’ve known for years that the rhythm method is problematic because it relies on people with regular [habits],” she said. “You have to literally take your temperature the same time every morning, first thing, and if you don’t, then you can’t believe it will be accurate.”

Streicher said that even if users were dogged about taking their temperature at the same time every morning, several seemingly simple factors could thwart the accuracy of the thermometer, like having a cold, going to the bathroom first, or having a sip of coffee. That’s why she has told patients trying to get pregnant as long as 15 years ago to throw out their thermometers.

“What we know about reliable contraception is that it can’t be user-dependent,” she said. “Long-acting ones like IUDs and implants will be the most reliable. The minute you rely on action, the efficacy goes down. So take something like this and you know the failure rate is gonna be sky-high.”

In its note announcing it was approving Natural Cycles, the FDA said that “women should know that no form of contraception works perfectly, so an unplanned pregnancy could still result from correct usage of this device.” But Streicher said that the company was spreading inaccurate information: It tells users when they can have unprotected sex, but Streicher said women can technically get pregnant at several points during their cycle.

“Sperm live longer than most people realize. Hearty sperm can hang out for 72 hours, while an egg can be viable anywhere from 48 to 72 hours,” she said. “Women also get into trouble when they see blood and think it’s their period, and they can then get pregnant at any time. You can’t promote not using contraception. You don’t just get a hall pass.”

Streicher said that she finds the idea of the app to have a “frightening political undertone.” She believes its messaging rings similar to the anti-contraception movement.

Natural Cycles users in Sweden have had unwanted pregnancies

The Natural Cycles app has already been linked to unwanted pregnancies. In January 2018, a hospital in Sweden reported that 37 women seeking abortions said they’d gotten pregnant while using the Natural Cycles app.

When Scherwitzl was contacted by Vox for comment for this story, a publicist from the company’s London-based PR firm, Madano, responded with a statement.

“One of the ongoing challenges with all forms of contraception is that there is always a statistical chance of unintended pregnancy, since no method is 100 percent effective,” the spokesperson said. “The effectiveness of Natural Cycles is supported by clinical evidence.”

Natural Cycles said it performed a study of the menstrual cycles of 22,785 women and found that the app was effective in preventing pregnancy 93 percent of the time. The company spokesperson said “typical use effectiveness takes into account all possible reasons why a woman could become pregnant while using the app.” Those reasons may range “from having unprotected sex on a red day, to the app wrongly attributing a green day, or the chosen method of contraception on a red day having failed.”

FDA spokesperson Deborah Kotz told Vox the agency was fully aware of the reports of Swedish women becoming pregnant while using the app: “We reached out to the Swedish authorities and feel that the information regarding the pregnancies in Sweden is consistent with our knowledge concerning the pregnancy risks associated with use of this device. No method of birth control is 100 percent effective, and we encourage health care providers and women to discuss all options for contraception to choose the best option for them,” she wrote in an email.

Kotz said the FDA knows that “an increase in the absolute numbers of unintended pregnancies is expected with a growing number of users.” Because the user base of the app will most definitely grow, and the effectiveness rate is 93 percent, unwanted pregnancies are inevitable.

The FDA reviewed clinical studies of the Natural Cycles app, Kotz said, and found that in a study of 15,570 women who used the app for eight months, 1.8 out of 100 women became pregnant while using the app. These numbers are actually better than mainstream methods of contraception: According to the Centers for Disease and Control and Prevention, the failure rate for oral birth control pills is around 9 percent, while condoms have a failure rate of 18 percent, and a diaphragm’s failure rate is 12 percent.

Then again, the CDC calculates that fertility awareness methods using calendars have a failure rate of about 24 percent. The most effective methods are implants and IUDs, which have failure rates of 0.05 and 0.2 to 0.8 percent, respectively.

The app isn’t intended for all women

In reality, the Natural Cycles app is not meant for use by everyone. Streicher said that the basal thermometer method relies on women with regular cycles, but “there’s plenty that can throw off your cycle, like stress, traveling, or lack of sleep.”

And the app isn’t necessarily for women looking to avoid pregnancy completely. In 2016, co-founder Elina Berglund said that the app is designed for women who are planning to have children at some point but merely want a break from hormones. The company also hired Kristina Gemzell Danielsson, a doctor and professor at Stockholm’s Karolinska Institutet, to do research on the product, and she said she wouldn’t recommend the app to women who want to strictly avoid the possibility of becoming pregnant.

This information is on the Natural Cycles website but is buried on an informational page for medical professionals. On the page, there’s a diagram showing doctors who the ideal app user is; the company says that patients who would be “devastated” by pregnancy shouldn’t be recommended the app. Still, the app is currently marketing itself on the iTunes app store page as a catchall “digital birth control.”

A spokesperson for Natural Cycles said its algorithm takes the typical life cycle of eggs and sperm into account. It’s also sensitive to “subtle patterns in a woman’s cycle” and so “if it sees something unexpected such as a higher or lower than expected temperature, it will err on the side of caution and give a red day.”

Still, the Swedish Medical Products Agency is continuing to investigate how those 37 women using the app got pregnant.

Many young women are learning about the app on social media

Like most tech companies, Natural Cycles relies heavily on social media ads and an influencer network to spread the word. But there’s a lot at stake when the risk is unintended pregnancy. And while pharmaceutical companies in America have a huge budget for advertising, Natural Cycles has a very specific, millennial-focused type of marketing strategy, which can get complicated. (Case in point: One of their investors and influencers is Sweden’s biggest lifestyle blogger, Isabella Löwengrip, of Blondinbella.)

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Olivia Sudjic, a Natural Cycles user who got pregnant when the app changed from green to red on the same day, wrote in the Guardian about first seeing the app’s ads on Instagram. She writes how she was taken in by the marketing, with its “glowing women reclining in Scandi bedrooms, all pale grey sheets and dappled light, brandishing basal thermometers,” but felt “colossally naive” because she’d “used the app in the way I do most of the technology in my life: not quite knowing how it works, but taking for granted that it does.”

Sudjic spoke with a few other women who got pregnant on the app, and one lamented that she, too, found it via Instagram but “didn’t spot the hashtag at the very end of the caption which said that it was a sponsored post.” She now feels bitter that she’d “been treated like a consumer, not a patient.”

Of course, influencers promote health products all the time; Clear Blue pays celebrities big bucks to promote its pregnancy tests; so do companies like Advil and Vicks VapoRub. Kim Kardashian, no stranger to shilling questionable products, got into hot water with the FDA in 2015 after she promoted a morning sickness pill without adequately addressing the risk factors of the medicine.

But Natural Cycles’ sponsored content posts on Instagram don’t always include important details, like that users should be okay with the fact that they could get pregnant, or that they absolutely need to have a regular routine of taking their temperature.

Academics have called for more transparency from apps like Natural Cycles, especially since, according to the British business magazine Campaign, 50 percent of the app’s subscriber growth comes from Facebook and Instagram marketing.

“Natural Cycles’ marketing materials ought to be entirely transparent, more clear than they currently are about the limitations of their app and pregnancy risks,” researchers at the London School of Hygiene & Tropical Medicine wrote in July. “Natural Cycles have understandably and effectively taken advantage of a less-regulated arena with few competitors. Natural Cycles has the opportunity to stake a claim as the leading commercial operator in this field, but to do so it must ensure that it provides absolutely full and frank information about the efficacy and limitations of its products.”

Streicher said that “preying on a healthy, fertile demographic like women in their 20s just seems crazy.”

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Natural Cycles told Vox it only works with brand ambassadors who are over 25, and that marketing to young women on social media “allows us to be targeted towards the women who may be suitable for and interested in using Natural Cycles — and, importantly, it allows us to exclude those who would not be suitable, such as younger audiences.”

While Natural Cycles is the first digital birth control app to get the FDA stamp of approval, this nod now makes room for others to join the club, so long as they follow the FDA’s process and provide data from clinical trials. This will no doubt lead to a flood of femtech companies rushing for this type of authorization.

Some companies have already touched on this. At TechCrunch Disrupt in 2016, Clue CEO Ida Tin said Clue’s artificial intelligence was strong enough to determine which days a woman needed to use condoms, and added that the company was looking into FDA clearance to market itself as an app that can detect diseases from user data. But she also stated that “we don’t recommend that people use Clue to avoid pregnancy without anything else.”

Not all companies will be as forthcoming about their capabilities, though, especially in the name of profit. The FDA has regulations about marketing, but as more companies will dive in for the approval, it will have to stay on top of these types of ads, as will the Federal Trade Commission.

Natural Cycles said it’s committed to being transparent and would willingly work with the FDA or FTC to regulate how it’s promoted. It also said it eventually wants to “partner with health care professionals.” The company, of course, must get doctors on its side first.