A contraceptive app which uses body temperature to track a woman's menstrual cycle is 93 per cent effective, a landmark study has found.
The findings, based on 22,785 women, even show Natural Cycles works slightly better than the Pill in preventing pregnancies.
The smartphone app, which is approved in the EU as a contraceptive, predicts the days of the month when a woman is most fertile.
On these, which are called red days, it advises couples use a condom to reduce the risk of an accidental pregnancy.
Since its inception in 2014, its manufacturers have claimed it to have a 93 per cent effectiveness rate. However, the trials were based on much smaller groups.
The findings, based on 22,785 women, even showed Natural Cycles works slightly better than the Pill in preventing pregnancies
Professor Kristina Gemzell Danielsson, based at the Karolinska Institutet, Sweden, was behind the results - which are the largest of its kind.
She said: 'Since more and more women are choosing natural contraception as an option, we welcome that there is a certified app with solid clinical evidence.'
'It shows the app is effective'
Dr James Trussell, based at Princeton University, New Jersey, also welcomed the research published in the journal Contraception.
He said: 'This is the first study to provide typical and perfect-use estimates of effectiveness of an app and shows the app is effective.'
Participants involved in the study had an average age of 29. Nearly 7 per cent went onto become pregnant.
The Pill is known to have 91 per cent effectiveness in preventing pregnancies. Other methods can be as low as 75 per cent.
Dr Raoul Scherwitzl, co-founder of the app said the results 'confirm the effectiveness of Natural Cycles, which is used by 380,000 people worldwide.
Women 'shouldn't rely' on a smartphone app designed to track their menstrual cycles, fertility experts claimed in July.
Studies have even found Natural Cycles, which plots ovulation on a calendar, to be as effective in preventing pregnancies as the Pill.
But it's unreliable, says Dr Mary Jane Minkin, a professor of obstetrics, gynaecology and reproductive sciences at Yale University.
She told LiveScience: 'Don't rely on something like this.'
Natural Cycles: The facts
Natural Cycles asks the user to digitally record their body heat each morning, which then produces an algorithm to build a fertility schedule.
This then predicts which 'red days', when the woman is most fertile, and advises her to use a condom to slash her risk of becoming pregnant.
In the days leading up to ovulation, a woman's body is flooded by oestrogen, which cools the body, compared with the post-ovulatory stage.
During that second stage of the menstrual cycle, after ovulation, the hormone progesterone warms the body, raising its temperature.
The two phases are separated by an ovulation day - when a woman is at her most fertile - when the body's temperature rises between 0.2 and 0.45°C.
The app, available in the Apple App Store and the Google Play store, works on the so-called 'rhythm method'. Around 250 similar apps exist.
What have previous trials found?
A previous trial of Natural Cycles found just five women in every 1,000 experience an accidental pregnancy within a year of using it.
This is compared three in every 1,000 for the Pill, researchers wrote in the European Journal of Contraception and Reproductive Health Care.
The results mean women can potentially avoid unwanted pregnancies, while also side-stepping the unwanted side-effects of traditional medicine.
The menstrual cycle consists of three phases, the pre-ovulatory phase, the ovulatory and post-ovulatory phase.
The cycle begins with the first day of menstrual bleeding (M) and takes on average 29 days, but the length can vary from woman to woman.
During the pre-ovulatory phase, the body is oestrogen-rich and cooler in temperature.
During the post-ovulatory phase the body is warmed as progesterone 'warms' it raising its temperature.
The two phases are separated by ovulation day, where the body's temperature rises by around 0.2 to 0.45°C.
A woman is fertile from five days prior to ovulation day (O), because sperm can survive in the body for between two to five days, NaturalCycles said.
Two days prior to ovulation, when there is a surge in luteinizing hormones (LH), a woman is at her most fertile
Due to the fact each woman has her own individual menstrual cycle pattern, NaturalCycles uses its unique algorithm to learn the pattern of each woman's ovulation, to 'identify her fertile window'.