Medical Examiner

Why You Should Not Trust Fertility Apps—Yet

The fertility awareness methods they use are real science, but the apps haven’t shown they can deliver the same standard of care.

A Natural Cycles app screen and a circular birth control pill pack.
Photo illustration by Slate. Photos by Natural Cycles and Thinkstock.

On Aug. 10, the Food and Drug Administration approved fertility awareness app Natural Cycles as a form of birth control, signaling that the agency believes there is enough evidence that the app is comparable enough to, say, the standard birth control pill for the prevention of pregnancy that it endorses women using it for that purpose. Natural Cycles is just one of a slate of smartphone applications that women can now use to measure their fertility. Dozens of apps help women to predict the days they’ll be fertile in a calendar month. Even though many of the apps don’t explicitly say that they should or can be used as birth control, many women use that information to prevent pregnancy. The majority of those who use the fertility tracker Daysy, for example, are trying to avoid becoming pregnant, according to Daysy spokesperson Aaron Knarr.

Fertility awareness, where women monitor when they are ovulating and have the highest chance of conceiving, can be a highly effective way to prevent pregnancy—when it’s used correctly. The problem is that these principles are difficult to translate into app form, and at the moment, there’s very little data even available that could be used to assess if apps, overall, are effective as a means of preventing pregnancy. But that hasn’t stopped the companies behind them from marketing to people who might not be familiar with the principles of fertility awareness. Problematically, these apps often don’t provide the education that people need to reach peak effectiveness using fertility awareness methods.

The fact that fertility awareness works, but fertility awareness apps might not, makes it a complicated topic for medical professionals to explain accurately to their patients, many of whom are likely looking for an easy way to prevent pregnancy. “[The apps] are taking something that has legitimate scientific backing but applying it in a way that isn’t necessarily scientifically sound, and without providing guidance on how to navigate it,” says Rebecca Simmons, a researcher and fertility awareness specialist at the University of Utah. There are a lot of misconceptions around fertility awareness methods used for contraception, Simmons says, because they’re so often conflated with the rhythm method, a largely ineffective system where women count the days of their menstrual cycle and predict the timing of ovulation and fertility. Fertility awareness is also often seen as the contraceptive method used by religious communities that do not allow hormonal birth control, which causes some researchers, patients, and health care providers to shy away from talking about it and recommending it.

The Centers for Disease Control and Prevention puts the failure rate for fertility awareness methods at 24 percent—but that calculation lumps the rhythm method together with other, more effective fertility awareness methods. The more effective methods use physiological signs from the body—basal body temperature (which goes up after ovulation) and cervical discharge (which changes in consistency throughout the cycle)—to track fertility throughout the month. Women chart and track their bodies’ signals, which gives them a read on when they can become pregnant and, if they’re trying to prevent pregnancy, when they should avoid sex or use a condom or other barrier protection.

A longitudinal study that followed 900 German women tracking both cervical mucus and basal body temperature over 20 years found that, under typical use, the strategy was 98.2 percent effective in preventing pregnancy. Methods that use only basal body temperature or only cervical mucus are less effective, but still provide better contraceptive efficacy than the rhythm method. Hormonal birth control pills are 91 percent effective under typical use, and 99.7 percent effective under perfect use, which means that typical use of fertility awareness might be even more effective than taking the pill.

Using fertility awareness as a contraceptive, though, requires that women learn how to track and interpret their temperature and cervical mucus, which can take time and investment, Simmons says. Many women take a class or work with a specialist to learn how to chart their fertility. It requires more work and attention than taking a pill every day. But for women who can’t or don’t want to use hormonal birth control, it can be an effective alternative method of preventing pregnancy.

It’s not easy to condense these methods into digital applications that do the calculations for you, says Petra Frank-Herrmann, who studies natural fertility in the department of gynecological endocrinology and fertility disorders at the University of Heidelberg. “The effective methods still demand self-observation by women,” she says. “There’s no easy shortcut.”

Simmons noted that most of the fertility apps available weren’t developed in consultation with the scientific community. “What you get is a lot of products without a lot of scientific backing. It’s not necessarily the fault of the tech industry or the fault of science, but there’s not been a lot of cross communication and there needs to be more,” she says.

Of the wide range of apps available, some adhere more closely to best practices in fertility awareness than others, says Summer Starling, a public health and clinical researcher. “Using an app to track a period and predict ovulation is not really a fertility-based awareness method,” she says. “Those are more fertility awareness lite—it’s not a formal way of using fertility awareness.”

Some apps are designed to facilitate more hands-on fertility charting and replace pen and paper with simple digital inputs all stored in the same convenient place. With those, women still interpret their fertility themselves. Other applications, though, perform the calculations based on provided data. “The app is telling you when your fertile window is,” says Chelsea Polis, a reproductive health epidemiologist at the Guttmacher Institute. “It’s using an algorithm to determine it for you.” But, she says, the efficacy of individual algorithms is under-studied—and that’s the format used by Natural Cycles, for example.

Reviews of the apps using various methods to track fertility show that there isn’t great evidence, overall, for their ability to prevent pregnancy. A 2016 report on the performance of fertility awareness–based apps found that the majority were not evidence-based, and the authors concluded that users should not rely solely on an app to avoid pregnancy.

Only a handful of individual apps have published data on their models and efficacy. Those results, however, have been under scrutiny by the scientific community. Studies of Natural Cycles, for example, report that the app is 93 percent effective in preventing pregnancy with typical use. That research, which was funded by the company, was strong enough for FDA approval, but it’s been criticized for being poorly conducted: Frank-Herrmann wrote in the European Journal of Contraception & Reproductive Health Care that the 2016 Natural Cycles study calculated the “perfect use” rate inappropriately, and that the precision and accuracy of the algorithm the application uses to identify the day of ovulation (based on basal body temperature) has not been established.

Similarly, Daysy, a fertility tracking device that pairs a thermometer with an app, boasts 99.4 percent efficacy, which was determined by a March study funded by the company that developed the app. According to Polis, though, the data on Daysy is flawed: The study did not include women who had used the app for fewer than 13 months, which knocked out a large number of users inappropriately; survey participation was low; and the data on use and pregnancies was collected retrospectively, rather than by tracking women while they used the app. Polis published a commentary on these problems in the journal Reproductive Health in June, and is calling for the study to be retracted.

“It is a normal process that scientific work is criticized by other working groups or scientists through ‘comments,’ ” wrote Niels van de Roemer, medical director at Daysy manufacturer Valley Electronics, through the company’s spokesperson. “We are in close contact to the Chief Editor of the Journal Reproductive Health regarding the case. We have submitted a clarification and have gone into the various points of criticism in detail.”

The quality of the evidence around fertility awareness apps is a particular concern because most people using them probably don’t have prior exposure to the science around fertility awareness methods or realize what they actually need to do in order to use them properly, Simmons says. Like other forms of birth control, it’s not for everyone; people with irregular cycles or endometriosis shouldn’t use fertility awareness, for example. The data on basal body temperature needs to be collected at the same time each day, and poor sleep, stress, and alcohol can make the results less accurate.

“A user might not know that travel can influence basal body temperature, so if you go on vacation and expect things to work the same way, you might be interpreting information incorrectly and putting yourself at a different risk of pregnancy,” Simmons says. Most of the available apps don’t provide users with the same quality of information around fertility awareness that a class, or learning from an expert directly, would, Simmons says. “Some do their due diligence, but it’s up to the apps to police themselves.”

Recent reports of unwanted pregnancies among women using the Natural Cycles app have received particular attention in light of its approval as a contraceptive by European and U.S. regulatory agencies. Alone, though, these reports don’t discredit the app; all types of contraceptives result in some number of unintended pregnancies. “There’s a natural backlash, and people say, well, that didn’t work for me,” Simmons says.

Even with the flaws in the efficacy studies, Simmons says it’s a good sign that the data exist for Natural Cycles and Daysy in the first place. “At least Daysy did something that we can criticize, and at least we know something,” Simmons says. “For some apps, there’s no information at all. For the couple apps that have a small number of publications, that’s a positive step forward.”

The problem is that these initial studies ought to be replicated in larger sample sizes and the research ideally ought to be conducted by scientists who have no investment in the apps’ success. But developing an actionable research base around fertility apps is difficult because the products come and go quickly, Starling says. “We can’t do a study of 10 apps because a study takes a year and by the time results are in, there will be 100 new ones,” she said. “We have to find a balance where we’re involving the people making the apps.”

That the FDA is entering the conversation around fertility awareness apps is a good sign, from a quality standpoint, Simmons says. The initial approval of Natural Cycles as medical device provides a road map for other fertility apps seeking their own signoff from the agency, and it gives the FDA more avenues for oversight of these products. But the app does not need to be prescribed by a doctor, and approval does not mean that clinicians will become more familiar with fertility awareness. This is worrisome when the apps are getting so much media attention. “We know that people have a hard time understanding risk, and they might think something is safer than it actually is,” she says.

In the meantime, women are still using these products and expecting them to work. According to a survey published in June in the journal mHealth, people who use fertility apps or are interested in using them in the future want the products to be science-based, with strong evidence to back them up; over 60 percent of respondents rated “based on proven scientific evidence” as highly important. That’s great until you remember that a number of the apps already on the market don’t clear that bar.

But it’s hard to assess which are which. Plus, there’s very little guidance on which app might be best for which needs; the market is very crowded, and often even doctors will find it difficult to keep up. That leaves women reliant on word-of-mouth, ratings, and ease of use, according to Starling’s study.

Natural Cycles also uses social media influencers to promote its app—which, from a public health perspective, is concerning, says Amy Hough, a reproductive and sexual health researcher. Using social media influencers might mean the company is targeting younger women through advertising, but younger women—who could have more variable cycles and schedules—aren’t necessarily the group that would be best served by fertility awareness methods, Hough says. Plus, the ads are somewhat misleading: “It portrays Natural Cycles as something that’s easy and straightforward to start using, but that’s not the case,” she says. The company also promotes its app using the effectiveness data from the studies researchers say were poorly conducted. This actually caused the United Kingdom Advertising Standards Authority to ban one of its July 2017 Facebook advertisements, saying that it gave the misleading impression that the app was as effective as other, highly reliable forms of birth control.

Daysy also continues to promote its product on social media using the 99.4 percent efficacy statistic, which Polis says is a problem. “You link back to the marketing language and the aggressive way that they’re promoting this device, [and] it just seemed like this perfect storm of people being sold a device that hasn’t been appropriately tested, and for which the manufacturers are using incredibly irresponsible language,” she said. Even more concerning: Women who have raised concerns about Daysy’s efficacy statistics in social media groups discussing fertility awareness have reported being blocked by the company or removed from Facebook groups. (Daysy representative Aaron Knarr said in an email that two to three women were removed after complaints of spamming and sharing private comments on other forums. “It is not normal protocol to debate a commentary on an independent study via a social media PR campaign,” he wrote to me. “This claim about lack of transparency on social media has no factual nor scientific relevance to the content of the original study.”)

The upshot of all of this? People turn to fertility awareness because they’re searching for nonhormonal options, or because existing products aren’t working for them. The apps have the potential to help meet those needs. But given the current concerns around the evidence for their efficacy and the lack of education, users should proceed with caution before abandoning other forms of birth control. At the very least, they should supplement the app with education about the method; Simmons, for example, says she uses fertility awareness apps and would recommend them to people who are interested, but she encourages users to work with another source to get the additional fertility education they might need. “It’s not enough to just download them and start inputting information and expect that they’ll work,” she said.