A new app approved in Europe claims to defy the old joke “What do you call a couple who uses the rhythm method?”

(Answer: Parents)

Natural Cycles is the world’s first regulatory approved app for contraception, offering couples an option besides condoms, the pill, and IUDs.

It received class IIb medical device status from a German inspection firm employed by the United Kingdom’s regulatory agency for healthcare products.

Now, women across the world can access the app. More than 150,000 users in 161 countries currently use it.

“I love that the hormones are gone and that I get to know my own body in a different way. I am more aware of my cycle and my own natural hormones,” Sara Flyckt, a London resident, told Healthline.

Flyckt used the app for two years to prevent pregnancy after using both contraception injections and several types of contraceptive pills.

“I never really felt that they agreed with me and the hormones made me very unstable,” she says.

Avoiding hormone therapy for contraception is exactly why Elina Berglund Scherwitzl and Raoul Scherwitzl, husband and wife creators of Natural Cycles, developed the app.

“I didn’t want to use hormones anymore, and I wasn’t exactly excited to use a condom every time, having been in a relationship,” Elina Scherwitzl, PhD, told Healthline. “We looked into natural family planning solutions, but there was nothing out there that made it easy and reliable to use.”

The Scherwitzls, who both have doctorates in physics, applied their math skills to the natural family planning method, developed an algorithm, and created the app they believe is safe and easy to use.

“We packaged it into a mobile app to make it user-friendly and scalable at the same time,” Scherwitzl says.

Read more: Which birth control method is right for you? »

How it works

Users who sign up for the app for a year receive a free basal thermometer to track their temperature.

The yearly subscription costs $4.99 per month. Access to the app is also available for $8.99 per month, but this plan doesn’t include a thermometer.

Anyone can have a free trial for one month.

Each morning, you take your temperature and enter it into the app. An algorithm then determines your fertility that day.

If the app determines it’s a green day, then you can have unprotected intercourse. If it informs you that it’s a red day, then you should use protection.

“I’ve heard and read a few comments that people seem to think that it’s tricky to have to take your temperature every morning, but it really couldn’t be easier,” Flyckt says.

She puts her thermometer on top of her phone since it’s the first thing she usually reaches for when she wakes up.

“That helps you to remember,” she notes.

Scherwitzl says the app’s method differs from traditional fertility awareness methods because the rhythm method leaves assumptions about when a woman ovulates, or requires her to analyze her body temperature on her own.

“This is very error prone as you don’t only have the human error at play, but also you have to use very simplified statistical methods as well as only analyze one cycle at the time,” Scherwitzl says. “Our algorithm takes all this away by doing more advanced calculations and simply returning red or green, thus removing the human error, and it is always extra cautious in the first months before it has enough data from this person to be able to safely give green days, without making any assumptions about the unique user’s cycle or ovulation date.”

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As effective as the pill?

A 2016 clinical study of women using Natural Cycles to prevent pregnancy, performed by the Scherwitzls along with the Karolinska Institute, showed the app was as effective as birth control pills.

The results reported a perfect use Pearl Index of 0.5 (99.5 percent effective) and a typical use Pearl Index of 7.0 (93 percent effective). This means five women per 1,000 would become pregnant per year due to the Natural Cycles app giving the wrong green day. Seven out of 100 women would become pregnant due to any reason.

“Other contraceptives, such as the hormonal contraceptive pill, are in similar ranges, while contraceptive methods that are inside your body such as the hormonal implant or the IUD have higher typical use Pearl Index rates as the human error is minimal,” Scherwitzl says.

Still, Dr. Jane Oh, an OB-GYN in Illinois, says a Pearl Index of 7 sounds unrealistic. She states that most OB-GYN textbooks suggest that the natural rhythm method has a Pearl Index of about 20. This means 20 per 100 women will get pregnant in a year when using that method.

“In real life, I would expect it to be much less effective in terms of preventing pregnancy. Especially if you have highly irregular cycles,” Oh told Healthline. “In the best case scenario, natural family planning with or without the app should work pretty well, but specific criteria must be met.”

Oh says the criteria includes a woman who has regular periods, and two disciplined adults with excellent self-control who are not devastated if they do become pregnant.

“In other words, a highly organized and motivated couple,” says Oh. “This is not the real world. That’s why the best forms of birth control to date are ones that do not depend on the person doing anything for years and years, like IUDs, sterilization, and Nexplanon implants.”

Scherwitzl agrees that the app is for couples in stable relationships who are willing to use protection on red days. Since the study was performed on women above the age of 18, she does not recommend anyone younger using the app.

Because Natural Cycles does not protect against sexually transmitted infections (STIs), Scherwitzl stresses that it “Should therefore only be used by stable couples when the woman knows and trusts her partner.”

Read more: What are the side effects of birth control shots for men? »

Can it help you get pregnant?

Oh says the natural rhythm method is effective in helping women become pregnant.

When Flyckt wanted to become pregnant, she used Natural Cycles to help plan her pregnancy.

“I did this for about one year and I am now pregnant in week 30,” she says.

Scherwitzl is currently conducting a third clinical study on the app’s effectiveness in helping women become pregnant by investigating what variables impact time for pregnancy and infertility. These include age, BMI, previous contraception, intercourse rate, and cycle regularity.

While the results have not been published yet, Scherwitzl says she can share some insight.

“What is clear is that we help users plan the most when they are less likely to have intercourse on the most fertile day by chance. This is especially users that have irregular cycles, early or late ovulation, or intercourse less frequently,” Scherwitzl says. “Previous contraception is also very important. If they used Natural Cycles to prevent pregnancy before, they usually get pregnant very fast once they switch to plan, as both the app and the woman have a good understanding of her cycle already.”