Why is my period late?

A period that has not arrived is one of the few health questions people ask at 2am. Most late periods turn out to be nothing, and what to do next depends almost entirely on whether pregnancy is possible for you this cycle.

When is a period considered late?

A period is late when it’s past the longest gap your own cycles usually run to, and the common rule of thumb is five days past the day you expected it. Almost nobody runs on a 28-day clock. The NHS gives a normal range of 21 to 35 days between periods, with 28 sitting in the middle rather than defining the edges, and the NICHD at the National Institutes of Health notes teenagers can range from 21 to 45 days and still be normal. So a period arriving on day 32 is late only if your own cycles usually run to 29.

You can’t know that baseline without a record of the last several cycles. A single long cycle is also weak evidence of anything. The NHS is direct about the ordinary case. The gap between your periods may vary each time, and periods that come early or late are what having irregular cycles means.

Count from day 1, which is the first day of real bleeding rather than spotting. Getting that day wrong by two days makes every cycle length after it wrong by two days, which is enough to invent a lateness that isn’t there.

If pregnancy is possible, test first

Pregnancy is the first cause on every clinical list of missed periods. It’s the only one where the answer changes what you do today, and the cheapest to rule out.

The NHS guidance on doing a pregnancy test is specific about timing. Tests are most reliable from the first day of a missed period. If you don’t know when your period was due, test at least 21 days after you last had unprotected sex. Home tests detect hCG, which the body starts producing around six days after fertilisation, so testing much earlier than that produces the confusing results.

A positive result is almost certainly correct. A negative result is less reliable, because it can mean you tested too early. If your period still hasn’t arrived a few days later, test again. Hormonal contraception doesn’t stop a pregnancy test working, and no method of contraception is completely effective, so being on the pill isn’t a reason to skip the test.

Why is my period late if I’m not pregnant?

Once pregnancy is ruled out, the NHS list of causes of missed or late periods is short and mostly unsurprising:

• Stress. The everyday kind counts. A hard month at work can move a period.
• Sudden weight loss, or being overweight. Both ends of the range disrupt the hormone signalling that starts a cycle.
• Doing too much exercise. It’s a common and often missed cause in people who have recently increased training volume.
• Hormonal contraception. The pill, the injection, and the IUS all change or stop bleeding by design.
• Breastfeeding. Periods often stay away for months.
• Perimenopause. This is the run-up to menopause, usually somewhere between 45 and 55. ACOG describes perimenopause as the phase where hormone levels and the cycle begin to change, and defines menopause itself as twelve months with no period. Our guide on tracking your period in perimenopause covers what the gaps look like and what to log.
• PCOS, now called PMOS. Polyendocrine metabolic ovarian syndrome, renamed in 2026, is one of the most common reasons cycles go long and unpredictable.
• Thyroid and other conditions. An overactive or underactive thyroid, diabetes and heart disease can all stop periods.

Almost all of these act slowly and show up as a run of odd cycles rather than one dramatic one.

How late can a period be?

A period can be as late as ovulation was. A cycle has two halves, and they are not equally flexible. The stretch after ovulation holds at around two weeks. The stretch before it is the one that moves. NICHD puts ovulation anywhere from 10 to 21 days after day 1.

So a late period usually means ovulation was late and the period followed it on schedule. Stress, illness, travel and a change in training load all act on that first half. A late period isn’t a sign that something is building up. The clock started later than usual.

When to stop waiting and see someone

The NHS advice for missed or late periods is concrete. See a GP if:

• you have missed your period three times in a row
• your periods have not started by the time you are 15
• you have missed a period alongside other symptoms such as weight gain or weight loss, tiredness, hair growth on your face, or dry or oily skin
• your periods have become irregular

Clinically, three is the number that matters. NICHD defines secondary amenorrhea as an absence of more than three cycles in someone whose periods were previously regular, and says that missing a period, when pregnancy, breastfeeding or menopause isn’t the cause, generally points to something else that needs finding. A GP will usually ask about your health and may run blood tests to find the cause.

None of that is a reason to wait three months in silence if something feels wrong. Severe pain, bleeding that is much heavier than usual, or a late period alongside symptoms that worry you all call for an appointment now rather than at cycle three.

What to bring to that appointment

The single most useful thing you can hand a clinician is a list of when your last several periods started. It’s the difference between saying your cycles feel off and saying the last four were 29, 41, 33 and 47 days.

Recording it as you go beats reconstructing it afterwards, because memory keeps the alarming cycles and drops the ordinary ones. Our guide on keeping a period diary covers what to write down and what to skip. If long or unpredictable cycles are normal for you, the guides on why periods go irregular and tracking with PCOS go further.

How Plum handles a late period

We make Plum, so this part is about our own app.

Plum predicts from a rolling median of your recent cycles rather than a plain average, so one unusually long cycle doesn’t drag every prediction after it. Once your cycles vary enough that a single confident date would be dishonest, it shows a prediction window instead of a day. So Plum doesn’t flag a period arriving two days after the middle of the window as late, because it isn’t.

Plum deliberately doesn’t guess why. It won’t tell you that you’re pregnant, and it won’t diagnose PCOS or a thyroid problem. It keeps the record straight so that you, or a clinician, can see the pattern that answers the question.

This is a record of something private, so we should say how we handle the data. Plum shows no ads and never sells personal data. It does require an account and syncs to our servers, encrypted in transit and at rest, so we won’t claim it stays on your phone. You can delete your account and everything in it from inside the app at any time. If you share a cycle with a group, the group sees a summary of your dates and nothing else.