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Apps promising to help women “take control” of their sex lives by predicting the days when they are fertile are putting users at risk of unplanned pregnancy by making misleading claims.
Millions of women in the UK – including 69% of 18-24-year-olds – have used smartphone apps that track their periods. Many also tell them their “fertile window”: the days when they are most and least likely to get pregnant.
But the quality of the data used to make these predictions varies drastically and is often limited and unreliable, experts warn. Most apps use just the dates of the user’s past periods and standard cycle information to predict when their next one will be – and, by extension, when they are likely to ovulate and be at their most fertile.
Despite this, apps that use only basic data are being widely promoted to women in the UK as a reliable form of contraception. An Observer analysis of fertility and period-tracking apps in the Apple and Android stores last week found some that are not registered as medical devices being described as “birth control” and claiming to be “as effective as condoms”.
One, called Period & Fertility Tracker, promotes itself as an “ovulation tracker” and says it is based on “scientific research” which “helps to determine the fertile days of the month, so you can either achieve or avoid pregnancy.” However, its predictions are based purely on information entered by the user about their period dates and length.
Another, called Cycles, says it is a “fertility and ovulation app” and promises “accurate” daily predictions of a high or low chance of getting pregnant. Its website includes a disclaimer that it is for “information only”. But its app description promises “no more surprises, worrying or feeling in the dark” and invites people to make the app their “reliable healthcare partner”. It is based on the standard days method, which is among the least effective contraceptive options.
Other apps offer the chance to “manage your fertility goals”, “postpone pregnancy” and “check your odds of conceiving each day”. One tells women it can “predict your chances of pregnancy” so they can “take control of their sex lives” and “feel good in bed”. Some have been downloaded millions of times. Almost all are unregulated, and many are based outside the UK.
The apps are being promoted amid concern about a rise in the proportion of women using “natural contraception” and giving up hormonal contraception. Last week, a study using data from the British Pregnancy Advisory Service (BPAS), published in the BMJ, said that 2.5% of women seeking abortions in England and Wales in 2023 reported that they had been using natural methods, such as fertility tracking, at the time when they fell pregnant – up from 0.4% in 2018.
Those who reported using no contraception when they became pregnant rose to 70% in 2023, up from 56% in 2018. While the study did not show a direct causal link, researchers said the small but significant rise in people using “natural” methods needed investigating further and raised concerns about the accuracy of apps predicting fertility windows.
The apps mostly rely on traditional fertility awareness methods which are primarily based on date-tracking. They have been used for centuries to estimate the chance of conception but can be risky due to the potential for human error and individual bodily fluctuations. The NHS says about 24 in 100 women in a year will get pregnant using “natural family planning” if they do not do it completely correctly.
Some apps also require users to input data daily about their body temperature or cervical mucus, which can increase the accuracy. Between two and 23 out of 100 people using natural contraception get pregnant each year, depending on the method they use, the researchers said.
Past research into apps that give fertility predictions has found they are generally of poor quality, with a 2017 review of 73 calendar-method tracking apps finding that none could accurately predict ovulation, and another, in 2016, finding that just six of 40 apps had perfect accuracy in predicting the user’s fertile window.
Even so, women are often given black-and-white predictions, telling them whether they are “fertile” or “not fertile” on a given day – which they might use to make decisions about unprotected sex.
Dr Janet Barter, president of the Faculty of Sexual and Reproductive Healthcare (FSRH), said: “Knowing when your periods are doesn’t tell you when you’re ovulating, necessarily – so you do need more than that for it to be effective.”
The findings have prompted calls for a clampdown by the regulator to tackle the “clear dangers” unregulated apps pose. The Medicines and Healthcare Products Regulatory Agency (MHRSA), which oversees contraceptive devices, says that “health apps designed to be used as contraception or support conception are likely to be considered medical devices”. This includes those that “claim to be directly able to make pregnancies more likely or to be able to prevent pregnancy”, which should “have a UKCA or CE mark to show they meet the requirements of legislation and are acceptably safe”. A spokesperson said all alleged breaches were investigated.
But many apps appear to be avoiding oversight because regulation relies on manufacturers self-reporting their product’s “intended purpose”. This means that period trackers with fertile window features are typically put in the same category as health and fitness apps, which are not generally classed as medical devices, rather than contraceptive devices, which are.
Currently just one app – which uses basal body temperature monitoring to make predictions – has been granted permission to be marketed as a form of birth control. Natural Cycles, which is widely advertised in sponsored posts by influencers on TikTok and Instagram, claims to have a 93% efficacy rate for typical use.
But even Natural Cycles is considered risky by many experts because it relies heavily on the user inputting their data regularly and accurately and abstaining from sex or using alternate protection on fertile days. While it has been approved for use as contraception in the US, in the UK a review by Nice in 2021 found there was insufficient evidence for it to be approved for use in the NHS.
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In July, the Society of Obstetricians and Gynaecologists of Canada (SOGC) raised concerns that the app’s effectiveness may be being overstated and warned users to “avoid unprotected intercourse, especially while initial cycles are monitored”.
Natural Cycles rejected the criticism, saying its evidence was robust; that since the Nice review came out, more research had been done; and that the SOGC lacked understanding about its product.
A spokesperson said typical use of the app was “comparable to other methods of contraception, both hormonal and non-hormonal”, adding that “while a direct, head-to-head trial comparing these methods has not yet been conducted in the contraceptive sector, Natural Cycles is classified in the same effectiveness category as methods like the pill”.
Dr Catriona McMillan, a lecturer in medical law and ethics at the University of Edinburgh, said the market for the apps was currently a “wild west”. “A lot of them are free to download, and a lot of them indicate the woman’s fertile window. The reason that’s worrying is that period-tracking apps are not a medical device at all. They’re not regulated at all, besides by the advertising and data regulators,” she said.
There was increasing evidence, she added, that younger people were “going off things like the pill, IUD and the coil”, partly due to concerns about side effects, problems accessing healthcare and misinformation on social media, and instead turning to period-tracking apps.
“But because they’re not a medical device, the data they use to make these calculations can be from one person, two people, 10 people. They don’t have to have an evidence base. So fertility windows are often incorrectly calculated. And when young people are relying on these, then they can get pregnant very easily because the algorithms aren’t always working.”
McMillan called for all apps offering fertility predictions to be required to register as medical devices. She believes apps, as a minimum, should be required to carry disclaimers that they must not be used for contraceptive purposes. But she said that, in reality, this might not be enough: “How often do you read the description of an app or go on the website?”
Dr Chelsea Polis, a reproductive health scientist and expert on digital contraception, said: “Given what we know about how inaccurate many fertile window estimations are, apps that are untested and insufficiently regulated can really put people at risk of unintended pregnancies. But many period-tracking technologies are excluded from a higher level of scrutiny because they’re not labelled by the manufacturers as a medical device.”
Barter, from the FSRH, said the rise in “natural contraception”, and people reporting using no contraception at all, raised bigger questions for women’s health.
In a 2024 survey by BPAS, nearly half of women reported facing barriers to accessing contraception, including long wait times and difficulty securing appointments, as well as cost. A significant majority – 84% – said they had switched their contraception methods at least once, with one in seven unhappy with the side-effects of their current method.
“I think we should all be more concerned about finding methods of contraception that women do feel comfortable with, rather than trying to persuade them to be more comfortable with what we’ve got,” Barter said.
