The most common reasons for a late period are pregnancy, stress, illness, changes in weight, eating or exercise, starting or stopping hormonal birth control, PCOS, thyroid problems, breastfeeding and perimenopause. Emergency contraception and some medicines can shift a period too. And sometimes a cycle simply runs long because ovulation happened later than usual, with no deeper cause at all.
Below: how to rule pregnancy in or out, how to tell which reason fits you, and when to stop waiting and see someone.
First, check that you're actually late
"Late" only means something compared with your usual cycle, not the textbook 28 days (most cycles aren't 28 days). If your cycles usually run 30–36 days, day 31 isn't late.
Count from the first day of your last period and compare with your typical range; the free period calculator does the counting for you. For how many days past due still counts as normal, see how late a period can be. This article is about the why.
Could it be pregnancy? When to take a test
If you've had sex since your last period, rule out pregnancy first, even if you used contraception. Timing matters, because a test taken too early can miss a pregnancy:
- Most home tests can be used from the first day of a missed period, per the NHS. Mayo Clinic adds that results are more likely to be accurate if you test after that first day.
- If you don't know when your period was due, the NHS suggests testing at least 21 days after you last had unprotected sex.
- Use your first pee of the morning. Mayo Clinic notes that urine is most concentrated then, which makes the pregnancy hormone (hCG) easier to detect.
In early pregnancy, hCG roughly doubles every 2–3 days (Mayo Clinic), so even a day or two of waiting helps.
Late period, negative pregnancy test: what now?
A negative test after your period was due is reassuring, but not always the final word. A common reason for a false negative is simply testing too soon, and with irregular cycles "too soon" is hard to judge: if you ovulated later than usual, your period (and a positive test) isn't really due yet. Mayo Clinic points out that irregular cycles make test timing harder for exactly this reason.
What the guidance says to do:
- Still no period after a negative test? Test again in a few days to a week (NHS, Mayo Clinic).
- Negative on a second test and still no period? The NHS suggests speaking to a GP at that point. A doctor can do a blood test, which may be more accurate than a home test, and look for other reasons your period has stopped.
If the second test is negative too, pregnancy becomes much less likely. Time to look at the other usual reasons.
The most common reasons for a late period when you're not pregnant
Here's what to consider, and how to tell whether each one might apply to you.
1. Stress
Yes, stress can genuinely make your period late. Mayo Clinic explains that mental stress can temporarily change how the hypothalamus (the part of the brain that directs your cycle hormones) works, pausing ovulation and periods. They usually return once stress eases.
How to tell: Think big or ongoing stress (a bereavement, a job loss, exams, a breakup), not an annoying week. Cleveland Clinic notes that brief, lighter stress tends to have less impact. A major event in the weeks before you'd normally ovulate makes stress a strong candidate.
2. Illness
Cleveland Clinic lists viruses and other illnesses among the things that can make a period irregular.
How to tell: If you were properly sick in the couple of weeks before your period was due, that alone could explain one late period.
3. Weight change, under-eating or intense exercise
Your body needs enough energy to run a cycle. When it doesn't get it, the hypothalamus can stop releasing enough of the hormone (GnRH) that kicks off ovulation. Doctors call this hypothalamic amenorrhea. Cleveland Clinic lists not eating enough, excessive exercise and stress as the usual triggers, often combined, and notes it's usually reversible. Weight change in either direction counts: the NHS lists both sudden weight loss and being overweight as causes.
How to tell: Have you started training hard, cut calories, or noticeably lost or gained weight recently? Don't just wait this one out: Mayo Clinic notes that low estrogen, which can come with long-term missed periods, can weaken bones. If food or exercise has started to feel out of control, please talk to someone.
4. A big change in routine: travel, night shifts, upheaval
Cleveland Clinic notes that disruptions or changes to your daily routine can affect your menstrual cycle. Long-haul travel, night shifts, moving house or weeks of short sleep all qualify, and they often come bundled with stress.
How to tell: A big shake-up in the weeks before you'd usually ovulate is a plausible explanation for a one-off delay.
5. Emergency contraception (Plan B and other levonorgestrel pills)
Can Plan B make your period late? It can. According to MedlinePlus (the US National Library of Medicine), after taking levonorgestrel emergency contraception it's normal for your next period to come up to a week earlier or later than expected.
How to tell: If you took it this cycle, a few days' delay is expected. If your period is more than a week later than expected, MedlinePlus advises contacting your doctor, who will probably suggest a pregnancy test.
6. Starting, stopping or switching hormonal birth control
Mayo Clinic notes that hormonal methods (some pills, the implant, the injection and hormonal IUDs) can stop periods altogether; copper IUDs typically don't. After you stop hormonal contraception, periods can take a while to return, especially after the injection.
How to tell: Did you start, stop or change a method in the last few months? If so, a late or missing period is common. Here's what's normal after stopping birth control.
7. PCOS
Polycystic ovary syndrome (which the NHS and Cleveland Clinic now call PMOS, polyendocrine metabolic ovarian syndrome) is one of the most common medical causes of late and irregular periods. With PCOS, hormone levels can stay flat instead of rising and falling the way they normally do (Mayo Clinic), so ovulation, and the period that follows it, may not happen.
How to tell: One late period isn't PCOS. A pattern of long or unpredictable cycles is the clue, especially with acne, extra facial or body hair, or weight changes. Here's what cycle lengths look like with PCOS.
8. Thyroid problems
The US Office on Women's Health says too much or too little thyroid hormone can make periods very light, heavy or irregular, and can stop them for several months or longer.
How to tell: Look for other signs that something has changed: unexplained tiredness, weight gain or loss, feeling unusually hot or cold, or a racing or slow heartbeat. A blood test can check thyroid function.
9. High prolactin
Prolactin drives breast milk production. When it's high outside pregnancy and breastfeeding, periods can become irregular or stop. Cleveland Clinic says the most common cause is a prolactinoma (a noncancerous pituitary tumor); some medicines and an underactive thyroid can also raise it.
How to tell: The telltale sign is milky nipple discharge when you're not pregnant or breastfeeding. Headaches or vision changes alongside missed periods are also worth getting checked (Mayo Clinic lists both among symptoms that can accompany them).
10. Breastfeeding
The Office on Women's Health notes that for some people, periods don't return until breastfeeding ends. Missing periods doesn't mean you can't get pregnant, though: Mayo Clinic notes that some people ovulate without having a period.
11. Perimenopause
As you approach menopause (which usually starts between 45 and 55, per the NHS), cycles often become irregular, with some periods arriving late or skipping entirely.
How to tell: You're in your 40s, your cycles have started varying more than they used to, and maybe you've noticed hot flashes.
12. Certain medications
Mayo Clinic lists some antipsychotics, cancer chemotherapy, some blood pressure medicines and opioids among medicines that can stop periods. Cleveland Clinic adds that some antidepressants and anti-nausea drugs can raise prolactin. If you suspect a prescription, talk to your prescriber rather than stopping it yourself.
What about antibiotics? Antibiotics aren't on these lists of medicines known to stop periods; the infection you're treating is a more likely culprit. Two do matter for another reason: the NHS notes that rifampicin (rifampin in the US) and rifabutin can make the combined pill less effective, and MedlinePlus says rifampin can make levonorgestrel emergency contraception work less well. If you've taken one, a late period is a good reason to test.
And sometimes, no clear reason at all
Your period usually arrives about two weeks after you ovulate (Cleveland Clinic puts this luteal phase at around 14 days). If ovulation comes a week late this month, so does your period, and you may never pin down why. One late period, a negative test and a normal next cycle is a very ordinary story.
Signs your period is coming (and why they can't rule out pregnancy)
The NHS lists these among the most common premenstrual signs:
- Cramping or bloating
- Breast tenderness
- Mood swings, irritability or feeling more emotional
- Tiredness or trouble sleeping
- Headaches, spots or greasy hair
- Changes in appetite or food cravings
The catch: many overlap with early pregnancy. The NHS notes that breasts can become tender in early pregnancy just as before a period, and that light spotting around when your period is due can be an early pregnancy sign. Tiredness and mood changes appear on both lists. So "it feels like my period is coming" can't tell you whether you're pregnant. Only a test can.
What to do right now
- Count the days from the first day of your last period and compare with your usual range (the period calculator helps).
- If you could be pregnant, take a test, ideally with first-morning urine, on or after the day your period was due.
- If it's negative and your period still hasn't come, test again in a few days to a week.
- Look back over the last month for the triggers above.
- Write down the date your period arrives. A few months of dates shows whether this was a blip or a pattern, and it's exactly what a doctor will ask for.
When to see a doctor
Book a non-urgent appointment if:
- You've missed three periods in a row (NHS, Mayo Clinic), or gone three months without a period and you're not pregnant or breastfeeding (Office on Women's Health)
- You've had two negative tests and your period still hasn't come (the NHS suggests speaking to a GP at this point)
- Your periods have become irregular, or your usual pattern has changed
- A missed period comes with other symptoms such as weight gain or loss, unusual tiredness, new facial hair, dry or oily skin, or milky nipple discharge
- You think under-eating or over-exercising might be behind it
Contact a doctor promptly (in the UK, your GP or NHS 111) if you could be pregnant, even with a negative test or no test, and have pain low down on one side of your tummy, vaginal bleeding or brown watery discharge, or pain in the tip of your shoulder. Together, these can be signs of an ectopic pregnancy. Sudden, severe tummy pain with dizziness or fainting is an emergency: call 911 (999 in the UK) or go to the emergency room.
Whatever you use to track (app or notebook), a few months of real dates turns "my period is late again" into something a doctor can work with. (We built Ebb for cycles that don't follow the rules: it predicts your next period as an honest range instead of a single date, and keeps your data on your phone.)
FAQ
Why is my period late if I'm not pregnant?
Common reasons include stress, illness, changes in weight, eating or exercise, a big change in routine, hormonal birth control changes and emergency contraception. Repeated late periods are more often linked to PCOS, thyroid problems, high prolactin, breastfeeding or perimenopause. Sometimes ovulation just happens later than usual.
Why is my period late with a negative pregnancy test?
Either you're not pregnant and another reason is at play, or you tested too early. Retest in a few days to a week. If that's negative and you still have no period, check in with a doctor.
Can you be pregnant with a negative test and no period?
Yes, especially if you tested early, or your cycles are irregular and you ovulated later than you think. That's why the advice is to retest. If home tests keep coming back negative with no period, a doctor's blood test can settle it.
Can stress make your period late?
Yes. Significant stress can temporarily change how the brain signals the ovaries, delaying or pausing ovulation, and a later ovulation means a later period. Brief, lighter stress tends to have less effect.
Can Plan B make your period late?
It can. After levonorgestrel emergency contraception, it's normal for your next period to come up to a week earlier or later than expected. If it's more than a week late, contact a doctor and take a pregnancy test.
Can antibiotics delay your period?
Antibiotics aren't among the medicines known to stop periods, but being ill can delay one. Rifampicin (rifampin) and rifabutin can make the combined pill less effective, so if you've taken one and your period is late, take a test. Most antibiotics don't affect home pregnancy test accuracy.
How late can a period be if you're not pregnant?
A few days past your usual range is common; the full breakdown is in how late can a period be. Three missed periods in a row is a firm reason to see a doctor.
What are the signs my period is coming?
Cramping, bloating, breast tenderness, mood changes, tiredness, headaches and spots are common premenstrual signs. Several also happen in early pregnancy, so they can't tell the two apart.