Apple Cycle Tracking: What the Health App Does, and What It Does Not

Your iPhone already has a period tracker, and for a lot of people it is enough. Apple's guide says the Health app logs period days, flow level, symptoms, spotting, basal body temperature and more, and shows a predicted next period and predicted fertile window days. Apple states that health data on a locked iPhone is encrypted, and that with two-factor authentication health data synced to iCloud is encrypted end to end and Apple does not have the key to decrypt it. What it cannot do is name a date nobody can know, and its Cycle Deviation notification is a reason to ask somebody, not an answer.

This is a review of the tracker most readers already own and never opened. Everything below about what it does comes from Apple's own support guides, quoted as such, because a feature list written from memory is how this genre gets things wrong.

What it lets you log

More than most people expect. Apple's guide says you can log menstrual cycle information in Health to get period and fertile window predictions, and that the log covers period days, flow level, symptoms, spotting, basal body temperature and more. So it is not only a calendar of start dates: the symptom and temperature entries are there, in the same place, without a second app.

Two shortcuts are worth knowing about because they remove the reason most logs get abandoned. Apple says an Apple Watch can be used to help log cycle information, and that on supported models Siri can log a period start. The entry that people forget is day one, always — and the version of that entry you can make out loud, from the bathroom, at seven in the morning, is the one that actually gets made.

What it shows you, and what that rests on

Apple says Health shows a predicted next period and predicted fertile window days, and a day you likely ovulated. That last one carries conditions in Apple's own documentation: it requires an Apple Watch Series 8 or later or an Apple Watch Ultra, and Apple says it is not available in all regions. If you have an older watch or no watch, that part of the feature is not there, whatever a comparison chart elsewhere tells you.

The prediction of your next period is arithmetic on your own history, and arithmetic on your own history is genuinely good at that. It is much weaker at the other thing. When researchers looked at ovulation in real cycles rather than in textbooks, day 15 was the commonest day at 27 per cent, then day 16 at 21 per cent, then day 14 at 20 per cent — and even inside a tidy 28-day cycle, the observed ovulation days spread across ten of them. Their conclusion was blunt: accurate prediction from calendar input alone is not possible.

That is not a criticism of Apple. It is the ceiling every calendar-fed tracker runs into, Apple's included, and it is the reason a fertile window shown as a block of coloured days is an estimate with a wide margin rather than a schedule. Nothing here is contraception, and a predicted window is not a plan.

If you want to see what your own numbers actually support, the cycle length calculator takes your last few start dates and works out your average and how much it moves, which is the number that decides how wide any prediction has to be. The period calculator turns that into a window with the uncertainty printed beside it instead of hidden behind it, and the cycle phase calculator shows where in the cycle today sits, with the same hedging attached.

The Cycle Deviation notification

This is the most interesting thing in the feature list and the most easily misread. Apple says a Cycle Deviation notification can flag irregular, infrequent or prolonged periods and persistent spotting. It watches the pattern you have been logging and tells you when the pattern has changed.

What it is not is a diagnosis, and it is worth saying that plainly because a notification about your body arriving unprompted on a Tuesday morning does not feel like a neutral statistic. It is a prompt. The useful thing to do with it is pair it with the thresholds a health service actually publishes.

The NHS says periods are irregular if the gap between them is under 21 days or over 35, that this is most likely when periods first start and when nearing the menopause, and that it is worth speaking to a GP if your periods are irregular or your normal pattern changes. Those two sentences do more for you than the notification does, because they tell you both what counts and what to do. Note the first part especially: if you are fifteen, or forty-eight, irregular is the expected state rather than a signal, and a phone telling you your cycles vary is telling you something your body already told you.

A deviation notification answers "has this changed?" It does not answer "why?", and it was never built to.

Where the data actually sits

Apple's published position here is stronger than most of this market, and it is worth quoting rather than paraphrasing. Apple states that when the iPhone is locked with Face ID, Touch ID or a passcode, all health and fitness data other than Medical ID is encrypted. It also states that with two-factor authentication turned on, health data synced to iCloud is encrypted end to end, and that Apple does not have the key to decrypt it.

Read that carefully, because it is doing two different things. The first sentence is about the phone in your hand. The second is about a copy on a server — a copy that Apple says it cannot read, which is a real and unusual claim, and which is still a copy. End-to-end encrypted sync and never-left-the-device are not the same architecture, even when the first one is honestly implemented. They fail differently, they depend on different things staying true, and they suit different people. That difference is the whole subject of the offline-versus-cloud comparison, so it is not relitigated here.

One more thing belongs in this section, because it is about judging the alternatives rather than Health itself. If you go shopping for a third-party tracker instead, you will read its App Store privacy label. Apple describes that section as a system in which the information is self-reported by the developer, and defines its terms narrowly: "Data Linked to You" means data collected in a way that is linked to your identity, and "Data Used to Track You" means data from the app linked with data collected from other companies' apps or websites and used for ads or shared with a data broker. A label is a developer's declaration in Apple's format. It is a good starting point and a bad finishing point, which is why the ten-minute privacy check exists.

Who it suits

The last of those is worth a sentence. Health is on every iPhone, so nobody learns anything about you from seeing the icon. That is a real privacy property and people rarely name it. It cuts the other way too: anyone who picks up your unlocked phone can open it, and it is exactly where they would look.

Where Athena sits

Athena is the app this site is being built for. It is in development, it is not in either store, and there is nothing here to buy or sign up for — so this is a description of how it is built, not something you can go and check today.

It takes the other option in this article. Periods, symptoms, notes and temperatures are written to a private database on the phone, predictions are calculated on the device, and there is no account and no server copy of health data. The app will send anonymous diagnostics — which features get used, crashes, the device model, never anything you log — to Firebase, and this website uses IP-anonymised web analytics; saying it sends nothing at all would be untrue. The honest cost of the on-device design is the one Apple's sync solves: there is no automatic backup, so if the phone is lost and nothing was exported, the history is gone with it.

Until it ships, the calculators above run in your browser and keep nothing. They will also tell you, in about two minutes, whether your own cycle is steady enough for any prediction — Apple's or anyone else's — to be worth much.