Natural Cycles

Is Natural Cycles safe?

Natural Cycles is a well-regulated app with sensible data rules, but it is only as safe as the contraception it provides, and that is the part to weigh first. In the study behind its 2018 FDA clearance, 6.5 in 100 women got pregnant over a year of typical use. With perfect use, the figure was about 1 in 100. The company's App Store listing puts typical use at 93% effective and in the same category as the pill.

On data, the picture is good. Natural Cycles is a paid medical device, says it never sells data, keeps cycle and sex data out of advertising, and offers an optional mode that unlinks your identity from your fertility records. We score it 68/100, ahead of Flo at 66, which once shared cycle data with Facebook and Google, and behind Oura at 76, whose ring it can use as a thermometer.

How well does Natural Cycles work as contraception?

The app reads your basal body temperature, from a thermometer, the company's own NC° Band, an Oura ring, WHOOP, some Garmin watches or an Apple Watch Series 8 or newer, together with your period dates and optional ovulation tests. An algorithm then marks each day green (not fertile) or red (use protection or abstain).

The FDA's De Novo summary, published when it cleared the app on 10 August 2018, rests on real-world data from 15,570 women aged 18 to 45, who used it for a combined 7,353 years. The FDA reported three rates:

Typical use is the figure to plan with. It counts every real-life slip, such as sex without protection on a red day or a condom that fails. The labeling also warns that women who came off hormonal birth control within the last 60 days have a higher risk, and that the app does not protect against sexually transmitted infections. The FDA has since cleared five changes through the 510(k) route, the latest on 17 July 2026.

The Swedish pregnancies and the UK ad ruling

In January 2018 Södersjukhuset, a Stockholm hospital, reported 37 unwanted pregnancies among women who had used the app, out of 668 women seeking abortions there over four months. By late March the Medical Products Agency (Läkemedelsverket) had received 108 reports, and it opened a supervisory case. It closed the case in September 2018, finding that the pregnancy rate matched what the company had published, and asked Natural Cycles to make the risk of pregnancy clearer in the app, according to STAT. There was no fine or ban.

On 29 August 2018 the UK Advertising Standards Authority upheld complaints from three people about a Facebook post seen in July 2017 that called the app "highly accurate" and a "clinically tested alternative to birth control methods", on both claims they challenged. The ASA said the claim leaned on perfect-use data when typical use is markedly less effective, and told the company not to exaggerate how well the app prevents pregnancy. We count that ruling as a small deduction. It is eight years old, and the current App Store listing gives the typical-use figure next to the perfect-use one.

What Natural Cycles does with your data

The privacy policy, last updated 7 August 2025, says Natural Cycles never sells personal data. It does not use fertility status, period, sex, symptom or pregnancy data to target ads, and it does not group users by that data. Research use needs your consent and is published only in anonymised form.

The catch is marketing. Natural Cycles matches hashed email addresses, IP addresses and device data with social networks to build "custom" and "lookalike" ad audiences. Apple's label lists contact info as used to track you, and in the US the policy carries the legally required line "We may sell your sensitive personal data", because state laws define a sale broadly enough to cover ad partners. Google Play's label says the app shares no data at all, which understates the policy. US users can opt out on the company's privacy choices page, which also honours the Global Privacy Control signal.

Accounts you stop using are kept for three more years unless you ask for deletion.

Is your data safe after Roe v. Wade?

After the US Supreme Court overturned Roe v. Wade in 2022, people asked whether cycle data could become evidence in states that ban abortion. Natural Cycles' answer has two parts. Its policy says it hands over identifying data only under valid legal process, contests requests where it has grounds, tells you first where it is allowed to, and discloses only what is required. And its optional Go Anonymous mode separates your identity from your fertility data, so that, the company says, "not even us at Natural Cycles" can link the two. Some features stop working in that mode.

A promise to resist a court order is not a guarantee. If your cycle data could ever matter in a legal dispute, Go Anonymous is the stronger protection.

How to use Natural Cycles more safely

1. Plan around the typical-use rate of about 7 in 100 a year, and talk to your doctor or a sexual health clinic if a pregnancy would be a serious risk for you.
2. Take your temperature as instructed and use condoms or abstain on every red day.
3. Turn on Go Anonymous if you live where abortion is restricted.
4. Say no to tracking on iPhone, and in the US opt out of "sale" and "sharing" on the privacy choices page.
5. Delete your account, not just the app, when you stop, or your data stays for three years.

How we scored it

The regulatory record (85) is the strongest factor: two medical regulators examined the app, and the only finding against it is an upheld UK ad ruling from 2018. Transparency and control (72) benefits from Go Anonymous. Privacy (60) is the weakest factor, held down by ad audiences built from users' contact details. Scams and harmful content (70) reflects the method's real failure rate and the gap between the 16+ App Store rating and the 18+ age limit, not any contact with strangers.