Your app put a little red number on the screen, and now it says you’re late. Before you panic, it helps to know how the app got to that date. Most of the time the app is late, not you.
Why does my period tracker say I’m late?
A period tracker predicts one date. It takes the cycles you’ve logged, works out a typical length, and adds that to the first day of your last period. The morning after that date, it starts counting days late.
Your cycles don’t work to one date. The NHS gives a normal range of 21 to 35 days between periods, and a 2019 analysis of 612,613 cycles put the average at 29.3 days, not 28. Even one person’s cycles shift from month to month. That’s because ovulation moves. NICHD puts it anywhere from 10 to 21 days after day 1, and your period follows about two weeks after ovulation. A late ovulation gives you a late period, right on schedule for that cycle.
So “2 days late” on the screen means “2 days past the middle of your range.” It doesn’t mean something has gone wrong.
How many days late is actually late?
Here’s a rough scale, assuming your cycles are usually fairly regular.
- 1 to 3 days late. Normal variation for most people. Most months nothing needs doing.
- About 5 days late. The usual point to call a period late. If you could be pregnant, take a test.
- A week or two late. This happens now and then, often after stress, illness or travel.
- Six weeks with no period. Count it as a missed period.
If your cycles usually vary a lot, stretch every line on that list. Someone whose cycles run 26 to 34 days isn’t late at day 31. The guide on how late a period can be goes through the causes in more detail. The late period calculator works out how late you are against your own range.
If you could be pregnant, test first
No period app can tell you if you’re pregnant. It only knows the dates you’ve logged. The NHS guidance on doing a pregnancy test says tests are most reliable from the first day of a missed period. If you don’t know when it was due, test at least 21 days after you last had unprotected sex. A negative test a few days early can be wrong, so test again if your period still hasn’t come.
If your app shows a low chance of pregnancy on some days, treat that as an estimate. It isn’t a form of contraception.
Why is my period tracker always wrong?
If the app misses most months, one of these is usually the reason.
- Missed logs. A period you didn’t log looks to the app like one very long cycle. That drags the average out and pushes later predictions back.
- Spotting logged as day 1. A day of light brown spotting before the real flow moves your start date a day or two early, and the next prediction moves with it. The guide to what counts as day 1 settles which day to log.
- Too little history. With one or two cycles logged, most apps fall back on the length you typed in at setup or on 28 days.
- Cycles that really do vary. If yours swing by a week, no calendar app can pick the day. It can only give you a range.
The research backs this up. A 2021 study of Clue data from about 186,000 people found predictions were off by a median of 1.5 days for people whose cycles barely changed. Error grew for everyone else, and the authors had to model skipped logs separately because they throw the numbers off so badly. Our guide to how accurate period tracker apps are has the rest of that research.
Why is my period tracker skipping a month?
Usually a period never got logged. Say your cycles are about 30 days and you forgot to log one. The app now sees a 60-day cycle. Some apps fold that into the average, and the next prediction jumps weeks ahead. Others leave the gap alone and keep predicting a period that already happened.
The fix is to add the missing period, even if you only remember the week. A rough date is closer to the truth than a 60-day cycle that never happened. If you really did skip a period, leave the gap in. That’s worth knowing, and the guide to irregular periods covers why it happens.
Why do different period apps give different dates?
Two apps fed the same dates can still disagree by a day or two, because each does the sum its own way.
- Some average every cycle you’ve ever logged. Others take the median of the last few, so one odd month counts for less.
- Some fall back on 28 days until they have enough history. Others use the cycle length you gave at setup.
- Some drop a cycle that looks like a missed log. Others count it.
- Apps that read temperature from a watch or ring can adjust the date once they see ovulation. Calendar-only apps can’t.
If two apps are more than a few days apart, check that both have the same periods logged. A gap in one of them usually explains it.
What to do when your app says you’re late
- Check your log. Make sure every period from the last few months is there and that each starts on the first day of real flow.
- Look at your range, not the single date. If your last six cycles ran 27 to 33 days, count from the longest one.
- If you could be pregnant, take a test from the day your period was due.
- Log anything unusual, like stress, illness, travel or a new pill. That’s what explains a long cycle when you look back.
The NHS page on missed or late periods says to see a GP if you’ve missed three periods in a row, if your periods have become irregular, or if you’ve missed one and have other symptoms such as weight change, tiredness or new facial hair.
How Plum shows a late period
We make Plum, a period tracker app for iPhone, so this part is about our own app. Plum predicts from the median of your recent cycles, up to the last six, once you’ve logged three. Until then it leans on the cycle length you gave at setup. A cycle more than a week longer than your usual one looks like a missed log, so Plum leaves it out of the median and one forgotten period doesn’t throw off every prediction after it.
From the day after the predicted start, Plum says your period is a number of days late. Insights shows how much your cycles usually vary, as plus or minus a number of days, so you can see whether 2 days late is inside your normal. If your cycles vary by more than about five days, Plum shows a window instead of a single date. It won’t guess why you’re late, and it won’t tell you whether you’re pregnant.
Plum shows no ads and never sells personal data. To compare it with other apps, see the most accurate period tracker app guide.
