Writing down when your period starts is genuinely useful, and it takes about two seconds a month. What it will not do at your age is predict a single certain date, and any app that shows you one is guessing. This page is health information for learning, and a doctor or a trusted adult can help with anything that worries you.

Why bother at all

Two reasons, and neither is about prediction.

The first is practical. Knowing roughly when to expect one means carrying a pad in the right week rather than every week.

The second is that your own record is the single most useful thing you can bring to a doctor. Doctors treat the menstrual cycle as a health signal in its own right, in the same way they use temperature or heart rate [ACOG]. "It came on the 3rd, then the 29th, then not for six weeks" tells a doctor far more in ten seconds than trying to remember in the room.

Why the window is wide, and why that is the honest answer

An app works out its guess from the cycles you have already recorded. If those cycles are 24 days, then 41, then 33, there is no single day that honestly follows from them — only a range.

At your age the cycles are supposed to vary. For teenagers, anywhere from about 21 to 45 days apart is within the usual range [ACOG]. A wide window is the app telling you the truth about what it knows.

That is why Femio shows a range instead of a date, and why the range is wider for younger users and gets narrower only when your own record supports it. It is also why there is no countdown and no "you are late" message anywhere in the app for you. Late only means something against a date that was certain, and there is no certain date here to be late against.

If your period does not arrive in the window, the app was not broken and neither were you. Read is my period normal for what the first years actually look like.

What is worth logging

Keep it small or you will stop doing it.

• The day your period starts. This is the one that matters. Everything else is optional.
• The day it stops, if you can be bothered.
• How heavy it was — light, medium, heavy — roughly.
• Pain, and whether it stopped you doing something. This is the note a doctor will ask about.
• Anything else you noticed — mood, spots, tiredness, sleep.

Missing days does not ruin it. A record with gaps is still much better than no record.

Keeping it from becoming another worry

Some people find that checking an app every day makes them think about it more, not less. If that is you, turn the reminders off, log the first day only, and close it.

The app is a notebook, not a judge. It has no opinion about you, there is nothing to keep a streak going, and nothing bad happens if you leave it alone for two months.

One thing to be clear about: tracking cannot tell you which days you could or could not get pregnant. Nobody's app can do that reliably, and it is especially unreliable when cycles vary, which is exactly what they do at your age.

Your data

What you log is yours. You can export it or delete it, and health data is not shared with advertising or analytics services. If you share a phone, it is worth knowing that anything on it can be seen by whoever else uses it — a screen lock is the simplest fix.

Words like cycle, lining and ovulation are explained in the glossary. How to track your menstrual cycle covers the same ground for adult readers, whose cycles are steadier than yours will be for a while.

When to see a doctor

Tracking itself never needs a doctor. What it does is make the appointment better if you have one. You or a trusted adult can arrange to see a doctor if your record shows:

• no period by about 15, or about three years after your breasts started developing
• periods that stop for more than about three months
• bleeding that soaks through a pad or tampon every hour or two, or lasts more than 7 days
• pain that keeps you off school
• bleeding between periods

Take your record with you, on your phone or on paper. If you would rather an adult came with you, ask one — that is what they are for.