Natural Cycles, the only app to be approved as a contraceptive, has proved 99 per cent effective in the largest study investigating it to date.

The startup had previously conducted a study of 4,000 women, which showed similar accuracy rates. It has now proven the efficacy of the app again after testing 22,785 women through a total of 224,563 menstrual cycles across a year, to calculate the app’s Pearl Index – the rate used to measure a contraceptive’s effectiveness. It found that if used perfectly – using protection such as condoms on red days – effectiveness is 99 per cent. Typical use, where people don’t use protection on the red days, leads to 93 per cent effectiveness, well above other natural family planning methods that rate at around 75 per cent and even the pill, which rates at 91 per cent.


Developed by former CERN particle physicist and co-discoverer of the Higgs Boson Elina Berglund and her husband Raoul Scherwitzl, the app uses daily body temperature readings to calculate when it is safe to have unprotected sex (signified as green days in the app calendar) and when to use contraception or abstain from sex (red days). This is possible because of the rise in progesterone levels after ovulation that makes women’s bodies up to 0.45C warmer than during the rest of their cycle.

“What we see in our data, which I find very promising, is younger women getting pregnant less when using the app even though they are more fertile, because they really don’t want to get pregnant,” Berglund says. “Slightly older women are those that get pregnant the most using the app.”

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This, she suggests, is because they may be considering getting pregnant and are using the app as a transition after coming off the pill. They may therefore be using it less 'perfectly' as they are not as concerned about falling pregnant.


Before the app was certified as a contraceptive, the Natural Cycles team found around two-thirds of women were using it as a contraceptive, and one-third to plan a pregnancy. Now, around 20 per cent are using it when trying to fall pregnant. Berglund wants to see women ultimately using the app throughout their entire fertile life, first as a contraception, then to plan a pregnancy once the app knows their cycle well, then after having a baby when their cycle is settling down again.

“People usually say what they like the most about the app is that it knows their body and helps them understand what’s going on inside,” she says. This could mean ovulation and fertility, but it could also help users predict and plan for days of the month they will experience the worst cramps or other premenstrual symptoms.

“70 per cent of our users come from hormonal contraception, and we see in our data that it’s very common that disrupts your cycle, sometimes for up to one year,” Berglund says. “The app is just as effective for these users, there are just more red days in the beginning. Generally, after one year the cycle stabilises and we see no long term effect from the pill.”

Natural Cycles aims to increase the number of green days in any given month with its annual algorithm update. “When we optimise the algorithm we make sure the effectiveness level is constant, but we try to increase the number of green days. We released a new version last week and managed to increase the number of green days by three per cent – that’s one day more in the cycle.” Extra green days can also be introduced with additional data, including a urine test that samples for the luteinising hormone, which spikes during ovulation.


The Natural Cycles team, who coauthored the peer-reviewed Contraception paper on the results with researchers including Princeton professor James Trussell, plans on following-up the study by looking at what impacts a woman’s decision to opt for the app over traditional methods. Berglund explains that a typical user is 29-years-old, in a stable relationship and often wanting a break from the pill. Natural Cycles also has data on patterns of usage – for instance, in countries where abortion is illegal, pregnancy rates using the app are far lower. Women in these countries tend to use the app conservatively, using additional protection on the red days.

One way Berglund hopes to encourage more women to use the app, is by simplifying the data collection process even further. “So far we haven’t found any hardware as sensitive as oral thermometers. There are several other companies trying to develop hardware to make it easier to track. I think our expertise is software, algorithms, and statistics, so we will make smart partnerships.”

Beyond a better sensor, making the algorithm smarter with more input data is also on the agenda. “Now Apple has included a bedtime feature together with an alarm, that tracks the user’s sleep. Sleep is something that can affect temperature so if we include that it can make it even better.”