Causes of Irregular Periods: Common Reasons and When to See a Doctor

It's day 44. Two pregnancy tests have come back negative. You've opened your period app more times than you'd admit, and quietly pinned the whole thing on the stress at work.  

Many times, irregular periods come down to a handful of things. PCOS and thyroid problems top the list, with high prolactin close behind. Eating too little for the exercise you do can throw a cycle off, and so can stress that drags on for months. Some phases of life do it on their own, like the first couple of years after your first period or coming off the pill. Pregnancy gets ruled out before anything else. Nearly all the causes of irregular periods show up on a few blood tests and one scan, and almost every one of them is treatable.

Visual: A woman sitting on her bed checking a period tracking app on her phone, looking puzzled  

How irregular is "irregular"?

Ask Google and you'll mostly get 21 to 35 days. Gynaecologists moved past that number in 2018, when FIGO (the International Federation of Gynecology and Obstetrics) redrew its limits. Now it's 24 to 38 days. Day one is the first day of proper bleeding. This NCBI summary has the full set of limits.

FIGO also limits how much your cycle can move, and that limit depends on age:

Say you're 30. Your last six cycles landed somewhere between 27 and 31 days, so you're fine, even if it never felt that way. Now picture one month at 26 days and another dragging on to 41: that 15-day gap makes the cycle irregular, though you'd probably have shrugged off each month on its own. Bleeding for more than 8 days is also outside the normal range.

These limits came from fairly small datasets, so use them to decide whether to book a visit and leave the diagnosing to a doctor.

Irregular periods symptoms, and what they hint at

Timing is the obvious sign. A period might turn up a week early, or not show at all for six weeks. Some women get brown spotting before the real bleed. Others notice the flow itself changing, soaking pads one month and needing barely a liner the next.

Your other symptoms usually point toward the reason.

What are the most common causes of irregular periods?

Here are the reasons for irregular periods, roughly in the order a gynaecologist checks them.

Pregnancy comes first

A home urine test is reliable from the day your period was due. If your cycles are long and you can't tell when that was, test about three weeks after the last unprotected sex.

PCOS

Polycystic ovary syndrome is the big one. It's the most common hormonal reason for irregular cycles in women of reproductive age, because higher androgen levels and insulin resistance stop follicles from maturing on schedule. Ovulation turns rare or random. The period follows.

You don't need cysts on a scan to have PCOS. Nor do a few follicles mean you have it. Doctors diagnose it when two of the three typical findings are present and other causes have been ruled out. More on this in our guide to irregular ovulation and its signs.

Thyroid problems and irregular periods

The thyroid is a small gland, but your ovaries listen to it closely. Too little thyroid hormone usually brings heavier periods that come closer together, though in some women it does the reverse. Too much tends to make them lighter and further apart.

It's also one of the easiest to miss. In a study, 44% of the 50 women seen for menstrual complaints had a thyroid disorder, most often subclinical hypothyroidism. Small sample, one hospital. Please don't read it as a national figure. It's why TSH usually comes first.

Subclinical hypothyroidism means TSH is raised while T4 is still normal. Your cycle can be the only clue. Treatment is usually a daily thyroxine tablet.

Stress and irregular periods

Stress does affect your cycle. Signals from the brain's stress response dampen GnRH, the hormone that starts the chain leading to ovulation. Ovulation gets pushed back and the period arrives late. Board exams or a death in the family can do this for a cycle or two.

Our view, plainly: stress explains one late period after a hard month very well. As an explanation for six months of chaos, it's weak. If your cycles haven't settled within two or three months of things calming down, get the other causes checked.

Eating too little or training too hard

Your brain keeps a rough tally of the energy coming in and going out. Cut calories hard before a wedding, or start training for a half-marathon without eating more, and it may decide this isn't the month to release an egg. The medical name is functional hypothalamic amenorrhoea. It's easily missed. She's often the fittest person in the room.

Gaining a lot of weight can upset ovulation too, even without PCOS. Fat tissue makes some oestrogen of its own and insulin tends to run higher, and both can nudge the cycle off course.

Prolactin, and what's in your medicine cabinet

Prolactin is the hormone behind breast milk. If it's raised when you aren't feeding a baby, ovulation stalls. Sometimes it's a tiny, harmless growth on the pituitary. Other times it's a prescription, since some antipsychotics and anti-nausea tablets raise prolactin as a side effect. Take a list of everything you're on to your appointment, but don't stop any of it on your own.

Life stages where it's expected

Some irregularity is simply a matter of timing. In the first couple of years after a girl's first period, the signals between brain and ovaries are still settling, so long or skipped cycles are expected. Coming off the pill can leave you waiting a few months for your own pattern to return, and if your cycle was patchy before you started, it may stay that way. Breastfeeding keeps prolactin high, which is why plenty of women don't see a period until they wean. Then somewhere in the forties perimenopause begins, and cycles usually get shorter before they start to stretch out.

Less common causes

A few rarer irregular menstrual cycle causes are worth knowing. Fibroids and polyps change how much you bleed more than when, and they can cause spotting mid-cycle. Endometriosis is mostly about pain. When the ovaries start winding down before 40 (premature ovarian insufficiency), erratic periods are often the first sign anyone notices.

Visual ref: Gynaecologist discussing blood test results with a young woman in a clinic 

Tests a doctor will usually suggest

Expect the first appointment to be mostly questions about your cycles, your weight and any medicines. A screenshot of your period app will save a lot of guesswork.

Blood work usually starts with a pregnancy test and TSH, with prolactin checked alongside. If PCOS is on the doctor's mind, you'll likely be sent for LH, FSH and testosterone, sometimes with a sugar test like HbA1c. If your periods are heavy, ask for a haemoglobin check as well, because anaemia is already common among Indian women. An ultrasound then lets the doctor see the ovaries and the lining of the uterus.

When should you see a doctor about irregular periods?

One strange month isn't a reason to rush to a clinic. Book a visit if your period has been missing for three months and you aren't pregnant, or if your cycles keep landing under 24 days or past 38. A swing of more than a week between your shortest and longest cycle is reason enough too.

Sometimes the bleeding itself is the warning. Periods that last longer than 8 days need a look, and so does soaking through a pad every hour or two. Spotting between periods or after sex should be checked, and so should irregular cycles that arrive with acne, new hair growth or a milky nipple discharge. So does turning 15 without a first period.

Trying for a baby? Go now. The usual one-year wait makes no sense here. Get care the same day if bleeding is heavy enough to make you dizzy, or if a positive pregnancy test comes with sharp pain on one side.

Even if pregnancy isn't on the cards, don't leave it for years, because a uterine lining that rarely gets its monthly reset can thicken abnormally over a long stretch.

Can you regulate your periods without medicine?

Sometimes, and it hinges on what's behind it. For women with PCOS who are carrying extra weight, losing 5 to 10% of body weight often gets ovulation going again. If the problem is too little food or too much training, it works the other way round: eat more, and ease off the training for a while.

The pill will make you bleed like clockwork, but that bleed isn't a true period. The pill switches your own cycle off, so it hides the irregularity and protects the uterine lining without touching the cause. That's fine, once someone has looked for the cause.

Supplements belong in the same place in the queue. Once a doctor has ruled out thyroid and prolactin problems, or is already treating them, some women like to add daily nutritional support. That's who we made Female Balance for. Shatavari and ashoka sit at its core, alongside vitamin D3 and L-methylfolate. It won't stand in for thyroxine or a proper PCOS workup. If you're pregnant, breastfeeding or on hormonal medicine, run it past your doctor first.

One last, very unglamorous suggestion: start a note tonight with the date each period starts and how many days you bleed. Three months of that list will tell your gynaecologist more than any single report you carry in.

Frequently asked questions

Can stress alone cause irregular periods?

For a month or two, yes. A rough patch can delay ovulation, which pushes your period late. If life has calmed down for a couple of months and your cycle still hasn't, something else is probably going on.

Can thyroid problems cause irregular periods?

Yes, in either direction, and one TSH blood test screens for both.

How late does a period have to be to count as irregular?

Depends on your usual pattern. A single period arriving four or five days late isn't a diagnosis. What doctors look at is the pattern over several months, especially cycles under 24 days or over 38, or a wide gap between your shortest and longest.

Can you get pregnant with irregular periods?

Lots of women with irregular cycles get pregnant. It just involves more guesswork, because ovulation is hard to pin down, so see a gynaecologist now instead of waiting out the standard year.