Why Is My Period Late? The Ordinary Explanations, and the Ones to Check

A late period is nearly always a late ovulation. Everything in the second half of a cycle follows the release of the egg, so when a period runs four days behind, it usually means ovulation ran four days behind — the delay sits in the first half of the cycle rather than in the second. That single fact explains most of the ordinary reasons a period is late, and it is why stress, illness, travel and a hard stretch of sleep can move a date that felt fixed. Pregnancy is on the NHS's list of common causes too, and no page that works from dates can tell you whether that is the answer this month; only a test can, and the honest timing for one is below.

This article explains. The period late calculator counts: give it your last period and your usual cycle length and it works out how many days past your expected date today is, and whether that is still inside the range your own cycles give. If you came here for the number, use the calculator and come back. If you came here because a number on its own did not tell you anything, keep reading.

Why the delay lives in the first half of the cycle

A cycle has two halves joined at ovulation. The first runs from the start of a period until an egg is released. The second runs from there to the next period. They do not behave the same way, and the difference is the most useful thing on this page.

In the study of 612,613 ovulatory cycles, differences in cycle length were caused predominantly by differences in the first half — the follicular phase. The second half was steadier: a mean of 12.4 days, with 18% of them under 11. So when a cycle runs long, the extra days are nearly always added before ovulation, not after it.

Two things follow. A period that is "four days late" is usually a cycle that started its second half four days later than usual, which is a different and much less alarming thought than a period that has gone missing. And the day you ovulated is not something a calendar knows, so the day your period is due is an estimate with a range around it rather than an appointment. In that same dataset, the mean movement in a woman's own cycle length was 2.6 days from one cycle to the next, which is the width of the ordinary blur before anything unusual has happened at all.

The common causes, as the NHS lists them

This is the NHS's own list of what commonly causes a missed or late period, in its own terms:

• being pregnant;
• stress;
• the start of the menopause, called perimenopause, usually between the ages of 45 and 55;
• polyendocrine metabolic ovarian syndrome (PMOS) — the hormone condition the NHS used to call polycystic ovary syndrome, or PCOS, and one of whose main symptoms is irregular periods or long gaps between periods;
• sudden weight loss;
• being overweight;
• doing too much exercise;
• using certain hormonal methods of contraception;
• breastfeeding.

Read it as a list of things that can move a cycle, not as a checklist about you. Nothing on this page and nothing in any app can tell you which of them, if any, is what happened this month — and several of them, PMOS in particular, are conditions a doctor investigates rather than things a reader diagnoses in herself at midnight.

Notice how many of them act on the first half of the cycle. Weight, exercise, illness and stress all work on the run-up to ovulation, which is exactly the stretch the large datasets say does the moving.

Stress, since it is what everybody reaches for first

It has a mechanism behind it rather than only folklore. Researchers report that prolonged or chronic stress in rats and in human females can block, inhibit or delay the preovulatory surge of luteinising hormone — the surge that triggers ovulation. Delay the surge and you delay everything downstream of it, the period included.

That is a real finding and it is also the most over-used sentence in this subject. Stress gets blamed for cycle length, for pain, for flow, for anything unexplained, and the research is much narrower than the blame. What it supports and what it does not, with the sample sizes attached, is what stress can actually do to a cycle. The short version: it is a reasonable explanation for a delay, and a bad reason to stop looking.

Pregnancy, straight

Most people who search this question have already thought about pregnancy, and dancing around it helps nobody. Here is what the NHS says, without softening.

Most pregnancy tests can be carried out from the first day of a missed period. If you do not know when your next period is due, do the test at least 21 days after you last had unprotected sex. And the two results are not equally reliable: a positive result is almost certainly correct, while a negative result is less reliable and can be wrong if the test is taken too early. So if a negative comes back and your period still has not arrived, testing again a few days later is the sensible move rather than a sign of paranoia.

The NHS also says that if you have a regular monthly menstrual cycle, the earliest and most reliable sign of pregnancy is a missed period. In the first few weeks there can also be a bleed similar to a very light period, with some spotting or only a little blood, which is called implantation bleeding — which is worth knowing mostly because it means a light bleed is not by itself an answer either way.

What no calculator can do is tell you the result. Dates cannot see a body. This site will work out when a test would mean something, and it stops there.

When one odd month becomes a pattern

One late period is a data point. The thresholds worth using are the NHS's, because they are specific enough to check.

• See a GP if you have missed three periods in a row, or if your periods have not started by the time you are 15.
• Speak to a GP if your periods are irregular, or if your normal pattern changes. The NHS counts periods as irregular when the gap between them is under 21 days or over 35, and notes that this is most likely when periods first start and when nearing the menopause.

That second line is the one people skip. A pattern that changed is information even when every individual cycle still sits inside the normal band. What the various definitions of irregular actually mean, and how far a cycle has to move before any of them applies, is in irregular periods, and when the variation is worth a conversation.

What to do with this month

Three things, in order of how much they help.

Get the count right. Late compared with what is a real question, and most people answer it against a 28-day cycle they never measured. The period late calculator counts the days past your expected date and then reads that count against your own shortest and longest cycle, which is the comparison that means something. That page is the counting half of this pair; this one is the explaining half.

Get your own numbers. If you do not know your shortest and longest, three or four recorded start dates will produce them, and the cycle length calculator turns those dates into an average, a range and a count of how many cycles it is working from. What that band ought to look like is covered in what counts as a normal cycle length.

Then wait or test, deliberately rather than by refreshing. A test has a date it becomes meaningful, and the pregnancy test calculator works that date out from whichever of your dates you actually know. Everything else on this page is a reason not to read a single late month as a verdict about anything.

One cause has a page of its own, because it comes with its own timetable and its own myths: periods after coming off hormonal contraception, in what happens after you stop the pill.