Will Sacks did not plan to go into the menstruation business. When he travelled from Toronto to Reno to attend his first Burning Man festival in August 2009, he only knew that he needed a change. At the age of 29, he was having a personal crisis. “I had forgotten that I wanted to be an entrepreneur,” he told me earlier this year. “I had forgotten that I wanted to create a company that could put a dent in the universe.” He quit his job as an energy efficiency consultant, shut down the small online business he had been running on the side, and booked a plane ticket to the desert.

Before beginning the drive to Burning Man from Reno airport, Sacks posted a message on Craigslist offering a ride in his rental car to anyone who needed one. A young woman named Kati Bicknell answered. Petite and pale, with thick brown hair, Bicknell looks like she stepped out of a pre‑Raphaelite painting. She exudes an intense, slightly mischievous, energy. Sacks comes across as calmer. He locks eyes when he talks to you, pausing every few sentences to check in: are you still with him? Does his optimism sound naive?

Bicknell had flown to Reno from New York, where she had a job at TED Talks. The drive to Burning Man became the beginning of a shared mythology. After the gathering ended, Sacks and Bicknell dated long distance, taking overnight buses between Toronto and New York. One weekend in November, they found themselves having “the birth control conversation”.

“We had been dating for three months,” Bicknell told me not long ago. “He said, ‘I don’t love wearing condoms and I was wondering whether you would be willing to go on the pill.’ I said … ‘No!’”

“I was relatively unenlightened at that point,” Sacks conceded, “and Kati got pissed.” Bicknell thought it was “bullshit” that the burden of birth control always fell on women. She suggested that her boyfriend get a vasectomy. Sacks balked. Then she brought up an alternative he had never heard of: the fertility awareness method (FAM). Fertility awareness involves regularly tracking certain physiological signs in order to determine when a woman can conceive and when she cannot.

Bicknell had always taken a strong interest in fertility. Her mother was one of over two million American women fitted with the Dalkon Shield in the early 1970s, a defective intrauterine device that caused hundreds of thousands of patients to suffer infections, miscarriages, and other serious problems; Bicknell grew up hearing how hard she had been to conceive. Ever since she began menstruating at 11, Bicknell had experienced highly irregular periods. No doctor ever investigated why; they simply put her on the pill.

In her 20s, Bicknell became concerned that the “fake period” the pill gave her every 28 days was masking a health problem that she should know about. A roommate gave her a copy of a book titled Taking Charge of Your Fertility, by a nurse named Toni Weschler. It is the bible of the fertility awareness method. By the time she met Sacks, Bicknell had already been practising it for three years.

“Fertility charting is this thing I’m already doing that can be used to prevent pregnancy,” Bicknell told Sacks that night in November 2009. “It’s just the kind of cool, nerdy thing you would be super into.” As she explained how it worked, Sacks became more and more stunned by his own ignorance.

“I had an engineering degree and thought I knew everything,” he told me. “I was blown away. On the one hand I was shocked and dismayed that I had been under this totally false understanding of how women’s bodies work. On the other, I was like, ‘Wow, there’s a side-effect free, hormone-free, form of birth control.’”

“This is a technology that can change the world,” Sacks thought. Bicknell agreed. That winter, they decided to ditch their jobs and move to Panama together. They rented a house near the ocean, took freelance gigs to pay the bills and spent the rest of their time in a hammock, poring over scientific studies and beginning work on their business pitch. When they returned north, they filed preliminary patents for an app that would make it easier for couples to practise fertility awareness.

They did not know how hard it would be to translate their ideas into a company. “We had no idea what we were doing,” Bicknell recalled, “and we didn’t even know it.”

In 1990, Carl Djerassi, one of the biochemists who created the oral contraceptive pill, predicted that the invention that had made him rich and famous could soon become obsolete. In an article in Science magazine, Djerassi explained that recent advances in his field had made it possible to track changes in blood hormone levels simply by taking samples of saliva or urine. This meant that, using simple cheek swabs, women could accurately predict the days when they could become pregnant. If they did so, taking the synthetic hormones that he had developed would become unnecessary. But Djerassi thought it was unlikely that fertility tracking would replace the pill. Why? Powerful pharmaceutical companies had a huge interest in pushing the pill. As for tracking, nobody had figured out how to make money from it.

For around 20 years, nobody did. But today, the market in “smart wearables” is booming. Biosensors connected to phones and other mobile devices make it easy to record, store, and analyse data about your body and behaviours. Apart from “activity trackers” – which record information about how many steps you take each day, how many stairs you climb, calories you burn, and so on – fertility trackers are the most frequently downloaded kind of health app in the Apple Store.

Fertility awareness apps have an enormous potential market: women from puberty to menopause. The industries that they could disrupt are huge, too. A 2013 study by Transparency Market Research estimated that in 2018, women worldwide will spend $23.3bn on contraceptives. Americans also spend $5bn a year on assisted reproductive technology treatments – at least some of which fertility awareness advocates say they could avoid, if they and their doctors paid closer attention to their bodies.

All fertility apps rest on a fundamental insight: there are externally observable physiological signs that can help a woman track where she is in her menstrual cycle, and provide clues about her overall health. A woman can only become pregnant during ovulation, the 24 to 48-hour period after one of her ovaries releases a mature egg cell and it travels down the fallopian tube to her uterus. Sperm can survive in the body for as long as five days. Therefore, there are only six days per month when having sexual intercourse could lead to conception. The trick is to figure out when that window is. Then you can plan sex, or plan to use barrier contraception, in order to achieve or avoid pregnancy.

Fertility awareness apps have an enormous potential market: women from puberty to menopause

The crudest form of fertility tracking gave the field a bad name. The rhythm method involves simply counting the days starting when your period ends, and then abstaining from sex around Day 14, the midpoint when ovulation is supposed to take place in the average cycle of the average woman. As the historian of medicine and technology Deanna Day has documented, the rhythm method first became popular in the United States in the 1930s, when the first studies of average menstrual cycle lengths were translated into English from German and Japanese.

As a form of birth control, the rhythm method is notoriously ineffective. To predict individual fertility reliably, it is not enough to extrapolate from past cycles. You must attend to physiological signs that indicate when you will ovulate. Two are especially important: basal body temperature (BBT) and cervical fluid. BBT refers to a woman’s temperature upon waking. In the days or weeks before she ovulates, a woman’s BBT will usually hover between 36.1C and 36.5C. Progesterone, the hormone that induces the ovary to release an egg, causes it to spike up to 36.6C or higher, where it stays until the end of her period.

Ovulation also triggers the cervix to produce new kinds of secretions. The wetness that you might notice on the crotch of your underwear at different times of the month follows a predictable pattern: dry, sticky, creamy, egg white. Then you bleed; then start at dry again. If you are trying to get pregnant, it is egg white you want – clear and resilient enough that you can stretch it an inch or more between your thumb and forefinger, before it snaps. On microscope slides taken at different points in the cycle it is possible to observe the ways in which cervical fluid dramatically morphs over the course of a given a month. Before and after ovulation, it honeycombs into a thicket, where sperm snag and stay, ensnared. On the days when fertilisation might occur, it stretches into tubes that look, and act, like tiny, high-speed rail tunnels.

If it is practised perfectly, fertility awareness can be almost as effective as the pill at preventing pregnancy. In 2007, a group of German scientists published a multi-year study of 900 women in the Oxford-based, peer-reviewed journal Human Reproduction. They found that, under conditions of “perfect use,” only 0.6% of the patients practising FAM became pregnant over the course of a year. But, given the chasm between perfect and average use, and the challenges involved in consistently observing and accurately interpreting physiological signals that vary from woman to woman, very few doctors endorse FAM as a form of contraception.

The tech entrepreneurs behind period tracking apps say that they can change this – and that by gathering unprecedented amounts of data about understudied aspects of how female bodies work, they can revolutionise reproductive medicine.

Facebook Twitter Pinterest Illustration by Bratislav Milenkovic

Given that roughly half of all humans get periods for much of their lives, it is striking how little we actually know about them. Few scientists have investigated why different women experience menstruation so differently. Why is it cripplingly painful for some, when it hardly seems to affect others? Why do some women bleed regularly, and others irregularly; some heavily, some not at all? What even is PMS? And why does it seem normal that hundreds of millions of people spend several days every month in pain?

Many societies have a deeply ingrained belief that female bodies are mysterious and exist primarily to produce children. Historically, medical science has downplayed the suffering of women, viewing it as either an unfortunate fact of life or less severe than women say. The taboo that still surrounds periods exacerbates the situation. Women often do not report problems they experience; when they do, doctors often dismiss them. As a result, countless women suffer with undiagnosed conditions such as endometriosis and polycystic ovarian syndrome for decades.

The rise of fertility tracking apps conjures the prospect of finally overcoming these blind spots. By collecting huge amounts of data, they may enable new insights into long understudied aspects of how female bodies work, and generate public health policies better tailored to their needs.

The origins of the modern fertility tracking movement lie far outside mainstream medicine. For generations, fertility awareness methods have been passed down in private settings, like forms of occult knowledge. Two groups with different, even directly opposing, agendas, have developed and propagated them: devout Christians and women’s health activists.

The precursor to the US organisation Planned Parenthood, the Birth Control Research Bureau, began encouraging women to track their menstrual cycles in the 1930s, in order to predict when they might be fertile – and when, therefore, to avoid having sex. In the late 1960s, the pioneering feminist healthcare providers who founded the Boston Women’s Health Book Collective taught “charting” as a means for women to dispel their shame about their bodies and free themselves from blind dependency on male doctors. The first edition of their bestselling book on women’s health Our Bodies, Ourselves, published in 1971, contained an entire chapter on the subject.

Meanwhile, Catholic doctors were also developing an interest in fertility awareness methods. In 1968, Pope Paul VI issued the Humanae Vitae encyclical, exhorting believers to practice “natural family planning”.

A network of Catholic doctors continued to do research on the menstrual cycle and to offer fertility awareness-based obstetric and gynaecological care to their patients, even as the pill, IUDs, and assisted reproductive procedures became mainstream. At many churches, couples who were preparing to be married would be directed by their priest to a parish member, who would teach them about natural family planning. At the same time, women’s health activists also continued to educate one another. Several institutions, such as the Center for Fertility Awareness in New York, consisted of one woman who met and trained other women, two or three at a time, in her apartment.

Because fertility awareness methods developed in these marginalised spaces, no institution ever standardised them. The names of slightly different techniques still work like dog whistles, conveying entire belief systems. A woman who says she practises “FAM” is most likely doing so for secular reasons. Someone who calls it “NaproTech” or “NFP,” or talks about the Creighton or Billings Method, is almost certainly motivated by religious belief (most likely Catholic). The agnostic trying to steer clear of the highly charged debates that break out among these camps might use “cooperative restorative care”.

In addition to helping individuals better understand their bodies, and demand better treatment, fertility tracking apps hold a lot of promise for large scale research. They could illuminate the causes of medical problems that have long remained mysterious. For instance, fluctuations in body temperature can reflect thyroid disorders, or the presence of a tumour. Subfertility, for instance, is a symptom, rather than a condition. The standard procedure used to address it – IVF – may enable a patient to become pregnant, but it does not eliminate the underlying cause. Do fluctuations in hormone levels affect subfertility? How about weight gain or loss, caffeine intake, sleep, stress (as reflected by heart rate), geographical location?

The mainstreaming of fertility awareness has the potential to improve women’s health, both at an individual level and for the entire population. But how do you move a century-old method, out of the living rooms of activists or church meetings, onto a smartphone? And once you do, can it be turned into a viable business?

In the beginning, by their own admission, Sacks and Bicknell made a lot of mistakes. Sacks re-mortgaged his house in Toronto for $50,000 Canadian and promptly wasted the money hiring developers who built software that did not work. In those early days, they were calling their company “Moonlyght,” a name that Bicknell now cites as further evidence of their ineptitude. (“Moonlyght! How were you supposed to Google that?”)

In 2012, having got married, at Burning Man, on the first anniversary of the day they met – and finally come up with a name that stuck: Kindara – Sacks and Bicknell moved to Boulder, Colorado. With a population under 100,000, the small city, nestled in the Flatiron Mountains about 35 miles north-west of Denver, has the most tech startups – and the most yoga studios – per capita of any city in the US. Techstars, a prestigious incubator, is based there, and Google has broken ground for a new campus in the city centre, which will soon bring 1,500 more employees.

Boulder also had a clear model for what Kindara was trying to do: FitBit. The activity tracker sells a range of branded devices – pedometer-like widgets – that users sync with its free app. FitBit launched in 2009, in San Francisco. But Foundry Group, a venture capital fund based in Boulder, led the first investment round of $9m in 2010. When FitBit made its initial public offering in June 2015, Foundry’s options, representing nearly a quarter of the company’s stock, were valued at $1.6bn. Brad Feld, the founder of Techstars, has remained an evangelist of activity trackers since. Intrigued by Kindara, he agreed to mentor Sacks and Bicknell.

“The FitBit for your period” was a good sales pitch. And it was accurate, to an extent. Like FitBit, Sacks and Bicknell planned to sell a device as Kindara’s primary source of revenue, while giving the tracking app away for free. But FitBit was a device from the beginning, whereas Kindara had to attract users in the hopes of selling them something later. Developing their widget – the Wink, a custom thermometer – would take several more years.

In the summer of 2012, Sacks and Bicknell launched their app. Finally, their vision seemed to be materialising. But, as the first 5,000 users downloaded Kindara, Will and Kati noticed something that surprised them. In questionnaires they filled out when they downloaded Kindara, the vast majority of users said that they were not interested in a substitute for birth control. On the contrary: about 75% were tracking their periods because they wanted to get pregnant. Fan mail started pouring in from devout Catholics. Sacks still sounds bemused by it. “We were all about making this tool to empower women. They were all talking about God,” he said. “I call it the strange coalition,” James Gardner, Kindara’s chief technical officer, told me. “Some [of our users] are super empirically grounded, some are into homeopathic medicine, some are religious, or experience side-effects on the pill.”

During their first two years in Boulder, Sacks and Bicknell hustled constantly. During one five-month window Sacks did 65 pitch meetings, flying in a loop from Boulder to San Francisco to New York to Boston and back, collapsing for a day and then leaving again. Bicknell recalled that when she pitched, investors were often “freaked out” by her talking about blood and mucus. “A lot of guys were just …” she did her best parody of a sneering bro-grunt – “‘Unnnnnhhhh.’” She was surprised to find that older female investors were often even more uncomfortable than their male peers, who tended to “just tune out”. (This kind of gender prejudice is not uncommon in the world of tech: when Apple released the Apple Watch in June 2015, the Health Kit it came with let you enter 150 different kinds of health data, including sleep duration and zinc intake. It had no slot for periods.)

As Kindara grew, the strain of running the company began to wear on Sacks and Bicknell. They had argued before, but after their app launched they began clashing constantly. They disagreed about everything. What custom features to add and which to leave out. Whether a fellow fertility awareness advocate had her heart in the right place and was worth collaborating with. The colour of this button. The style of that font.

They tried to stay civil. But as they raised money and the number of Kindara users grew, things between them grew worse and worse. “Spend a year trying to keep your employees from seeing your wife cry,” Sacks said, shaking his head. “We would shut ourselves in the conference room and yell at each other. I kept telling myself that the only way out was through. By that point I had raised a million dollars. I felt a responsibility to my investors. I could not just shut it down.”

The launch of the Wink thermometer met one delay after another. (“Hardware is hard,” the saying goes.) Both Sacks and Bicknell were fanatical about the design: the Wink had to be something that looked handsome and discreet sitting on a woman’s night table. It had to have the right weight and feel in the palm of her hand. Bicknell felt strongly that it should buzz, not beep, when it finished taking your temperature. A beep sounded too much like an alarm clock, and was liable to wake up anyone with whom you happened to be sharing your bed. Designers know that these tiny choices can make or break a product.

Meanwhile, external pressures were mounting. By 2013, the fertility tracking “space,” as tech investors tend to say, was “heating up”. In August, Max Levchin, the co-founder of PayPal, launched a fertility tracking app called Glow. Within two years, the company had raised $23m in venture capital; users have now reported more than 100,000 pregnancies. Clue, another competitor, launched in Berlin. By the end of 2015, it had raised €10m and acquired 2.5 million users in 180 different countries.

Kindara continued to grow, adding hundreds of thousands of users. But by the end of 2013 Sacks and Bicknell were on the verge of divorce. At home they avoided each other, sleeping in separate rooms. “The company was started out of our relationship and our love for each other,” Will said. “But we were fighting and hating each other. I remember feeling so trapped.” It became clear that they could continue working together or they could stay married. They decided that one of them needed to leave Kindara.

They agreed that whoever stayed should become CEO. One of the advisers to the Kindara board brokered an agreement between the couple, and after helping Sacks search for a female president and COO to replace her, Bicknell departed in February 2014. Today, she expresses no doubt about her choice: “I was ready to leave. I had burned myself right out.” She has launched a new company, selling an organising product called Cordsnaps. Still, sometimes notes of wistfulness creep into her voice. “It took six months before I felt like doing anything,” Bicknell told me. “I didn’t leave the house.”

Facebook Twitter Pinterest Illustration by Bratislav Milenkovic

The founders of period tracking apps are all driven by faith that mobile technologies will revolutionise healthcare. But each of them sees that future shaping up a little differently. Kindara wants to make money by selling devices. Others are betting that they can find ways to monetise the information users provide.

Glow, for instance, was created inside a tech incubator called HVF, which Max Levchin started in order to “search relentlessly for opportunities that create value by leveraging data”. When he launched the Glow app at the D: All Things Digital conference in May 2013, Levchin told the audience that infertility was “an information problem”. “It can be solved by applying data to this issue,” he said. He then proceeded to create a Glow profile for his wife, logging into her Facebook account and entering information about their sex life and her menstrual cycle.

In order to collect more and more information from ever more precisely differentiated users, Glow launched a series of different apps, all free to download. The original Glow targeted women trying to conceive. The company then released Ruby, subsequently renamed Eve by Glow, to provide information on contraception and STDs. Glow Nurture is for women who are pregnant. In April 2015, Glow introduced a feature for men. The latest addition to the line-up, released in late February, is Glow Baby, which allows parents to track their newborns’ progress toward 101 specific developmental stages.

Glow has experimented with several tactics for generating revenue. They started by selling a novel kind of insurance. Glow asked users to commit to using the app, and to pay $50 per month. If they did not conceive after 10 months, Glow would draw funds from the pool to help cover part of the cost of IVF. The company also made a deal with a pharmacy chain, Walgreens, which would remind users of the Glow app who took birth control to refill their prescriptions at Walgreens pharmacies. More recently, Glow has established partnerships with two of the largest networks of fertility clinics in the US: Shady Grove and Boston IVF. In return for consultations with doctors, who encouraged patients to use Glow, the company developed bespoke versions of the app for both.

However, Glow’s primary strategy seems to be the strategy of companies such as Google and Facebook: gathering as much data about user behaviour as they can, and then figuring out what they will do with it later. It therefore raises the same uncomfortable question that Google and Facebook do: what happens when your user is your product?

“People trying to get pregnant are a very marketable segment,” Karen Levy, a professor at Cornell University who focuses on monitoring technologies, told me. Pregnant women start needing things they never needed before: tubs of lotion, prenatal vitamins, diapers. Long-held brand loyalties come up for grabs, and shopping habits shift. “So advertisers already know something huge about you if they know you are using a fertility app,” Levy said. Some users might feel reservations about volunteering so much data, if they understood just how valuable it was, and how it could be used to target them.

In the US, strict privacy laws govern how entities such as hospitals share information about their patients. But these do not apply to mobile health apps. The latter fall under consumer privacy laws, which offer much lower standards of protection. Questions surrounding data security are especially fraught because the information involved in fertility tracking is so intimate. For instance, Glow asks for information about the sexual position that a user was in when her partner ejaculated.

Levy stressed that, with interpersonal apps, the risks to privacy go beyond the ways that companies might use the information. “The people building these systems tend to think about privacy in terms of things like cybersecurity and data breaches. But they might not think enough about the privacy risks that might arise within intimate relationships.” Many trackers include features for “partner integration,” updating a man about his female partner’s menstrual cycle. (Apps devoted exclusively to this function have come and gone. The most recent to launch in the UK, Fredrick, appeared in late January with the strapline: “Don’t hate her. Navigate her.”)

But what if a woman does not want her partner to know everyone she is having sex with? Or to bring her water, if she has not logged enough glasses that day? You may not want your man to anticipate your menstrual cramps.

Fertility trackers can clearly improve patients’ ability to manage information and to communicate with their doctors and partners. They may help to dispel the sense of secrecy and shame surrounding women’s bodies and to create spaces online for discussing common problems such as endometriosis or infertility. But new technologies can also serve to reinforce very old beliefs. Like, for instance, that a man should monitor the body of his female partner. Or that women should be fixated on their fertility in the first place.

Many writers and scholars have documented how emotionally debilitating the process of struggling to become pregnant can be. It is easy to imagine that, for a woman struggling to conceive, having a FitBit for her period could increase the anxiety and obsessiveness that so many women in her position already experience. One patient I interviewed at Reply ObGyn, a clinic in North Carolina that provides fertility tracking-based care, began to weep when she looked at her iPhone. She had been telling me, in a hushed voice, about her three miscarriages. With her eyes lowered, fixed in a far corner of the room, she explained that she had deleted all of her tracking apps after the last pregnancy she lost. Seeing them made her too sad.

Kindara hopes to sidestep these troubling questions about selling user data by making devices their source of revenue. And Will Sacks insists that technology will soon transform healthcare for the better. “Right now people have relationships with doctors,” he said. “In the future it will be with a brand. We will have sensing devices that cross-reference data about our blood, our interstitial tissue, our genes, our saliva and insights will be fed back to us. Analysis will be done through the phone.” Of course, this seamless future is easier to imagine than it is to build.

Kindara is moving ahead smoothly for now. In January, the app was downloaded for the millionth time. And in the last days of February, after weeks of shipping delays, the Wink thermometers arrived in Boulder from Thailand. This first batch was small: just 12 Winks for staff members, and 30 destined for Kindara users who had requested to be among the first to try them out. More than 9,000 other users, from Chile to Myanmar, have already preordered the finished product.

The Wink comes packaged in a sleek black rectangular box that looks like a cardboard clutch, with a magnetised clasp. The device itself is oblong, slightly flared at each end, like a cigarette lighter with a cinched waist. To use it, you tap its head twice with your index finger, then pull the thermometer out of its sheath, like a wand of mascara. When you tap its head, it activates the app on your phone, which then invites you to sync the devices. Your phone or Apple Watch will then alert you every morning when it is time to check. Hold the Wink under your tongue and the data is automatically entered.

In the future, analysis of our blood, our genes, our saliva will be done through the phone

I visited Kindara in early March, on the first working day after the shipment of Winks had finally arrived. The office has the feel of a warehouse with homely touches – framed pictures of staff and board members, colour-coordinated to reflect their roles, ombré green-and-blue Ikea rugs, the inevitable boxes of granola bars and other healthy snacks. It is a no‑shoes office; most people shuffle around in brightly coloured socks. Through the large windows on one side of the office, I watched the slight wind push clouds, silently, across the vast sky toward the mountains.

The mood on the day I visited was convivial and upbeat. The morning started with a “sprint meeting”: an intense session with Sacks, the COO Sally Hatcher, and the engineering staff, to set goals for the next “sprint” – usually a one or two-week stretch. As the meeting unfolded, the rest of the team hung out, drinking their morning coffee and settling into their laptops. Scott Shambo, the supply chain manager, joked that his two young daughters had been “product testing” the new Winks all weekend. As the staff busied themselves, a plucky Pomeranian named Peejay pranced around the office. The dog, who belongs to one of the company’s two engineers, Shannon Bertucci, is listed on Kindara’s website as VP of Morale.

Staff at Kindara are sincere and passionate devotees of tracking culture. Over lunch, a group of them spontaneously started talking about their magnesium levels. Bertucci, a 27-year-old former army engineer with a sharp purple-on-peroxide-blonde bob, confessed that she could not wait to get her Looncup, a Bluetooth-enabled menstrual cup, which measures and analyses the blood that it collects. (Bertucci was an early funder of Looncup’s Kickstarter campaign.)

All dozen staff members had been given Winks. In free moments, they kept walking over to where they were charging and pulling them out to play with them. At one workstation, a laboratory-grade, printer-sized tank of water had been set up. The water in it purred, held at a consistent 36C. Team members periodically lowered a Wink into it to test that it read the temperature correctly. They tapped and slipped their Winks under their tongues, peering at their phones with intense concentration and starting with pleasure when their Winks buzzed.

Main illustration by Bratislav Milenkovic

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