Irregular periods: what counts, the usual causes and the blood tests that sort them

Most irregular periods come down to irregular ovulation. A period arrives about two weeks after an egg is released, so when ovulation is late, skipped or random, the bleed is too. The useful question is where the chain that drives ovulation (brain, pituitary gland, ovaries) has gone off, and a handful of blood tests can usually answer that.

What counts as an irregular period?

The International Federation of Gynecology and Obstetrics (FIGO) sets the reference numbers most doctors use. A normal cycle, counted from the first day of one period to the first day of the next, is 24 to 38 days long, and bleeding lasts up to 8 days ¹. Few people run exactly 28 days: in more than 600,000 tracked cycles the average was 29.3 days and only about 13% were exactly 28 ⁸. FIGO calls cycles regular when the shortest and longest cycles in a year differ by 7 days or less between ages 26 and 41, and by 9 days or less at 18 to 25 and 42 to 45 ¹. A 26-day cycle followed by a 31-day cycle is regular. A 26-day cycle followed by a 40-day cycle is irregular.

The 2023 international PCOS guideline uses slightly different numbers when it asks whether irregular cycles are a sign of polycystic ovary syndrome (PCOS). More than 3 years after a first period, cycles under 21 or over 35 days, or fewer than 8 periods a year, count as irregular; more than 1 year after a first period, any single cycle over 90 days counts ². The two definitions overlap for most people. They disagree in two narrow strips, 21 to 24 days and 35 to 38 days, and those are the zones where a doctor looks at the whole picture.

Are irregular periods normal? It depends on your age

Irregular cycles are expected at three points in life: the first years after periods start, the years before menopause, and after pregnancy. In between, a persistent change is worth a look.

Teens

In the first few years after a first period, cycles of 21 to 45 days are typical while the brain-ovary signalling matures ⁴. A check is warranted for cycles shorter than 21 days or longer than 45 days, any single cycle longer than 90 days, periods lasting more than 7 days, or no period by 15 or within 3 years of breast development ⁴. The American College of Obstetricians and Gynecologists (ACOG) treats the cycle as a measure of health in its own right:

By including an evaluation of the menstrual cycle as an additional vital sign, clinicians reinforce its importance in assessing overall health status for patients and caregivers.ACOG Committee Opinion 651, Menstruation in girls and adolescents, 2015 ⁴

20s and 30s

Between the early 20s and about 40, cycles are at their steadiest. Irregular periods that last more than 3 to 6 months in this age band usually have a findable cause, most often PCOS, under-fuelling, thyroid disease or high prolactin.

40s

Perimenopause is the commonest reason for new irregularity after 40. The STRAW+10 staging system marks the early menopausal transition as a persistent difference of 7 days or more between consecutive cycles, and the late transition as a gap of 60 days or more without a period ⁵. Hormone tests are less help here than the calendar, because FSH and estradiol swing from week to week. As the perimenopause page puts it: "A single normal FSH therefore cannot rule perimenopause out, and a single high one cannot date it." Once periods have stopped for 12 months, the menopause page takes over; if periods stop before 40, that is a different question (see premature ovarian insufficiency below).

After pregnancy, while breastfeeding and after stopping the pill

Breastfeeding raises prolactin, which holds back ovulation, so periods may stay away or come irregularly until feeds drop off. After stopping the combined pill, cycles usually settle within a few months. If periods have not returned by 6 months, see a doctor to look for another cause, because the pill can hide PCOS or a thyroid problem that was there all along. The birth control and blood tests guide covers which results the pill changes.

What causes irregular periods?

Ovulation depends on a chain, and each cause breaks a different link. That is why blood tests can tell the causes apart.

More on the causes above: hot flashes, overtraining syndrome, hyperthyroidism, high prolactin, low estradiol and the birth control and blood tests guide.

Fibroids, polyps and endometriosis change how heavy or painful bleeding is, and can cause bleeding between periods, but they rarely change the underlying cycle length. If the timing is regular and the bleeding pattern has changed, the cause is more likely in the womb than in the hormones. If your cycle is regular but the days before it are hard, see PMS; for the wider hormone picture, see hormonal imbalance.

Blood tests for irregular periods, and the order to run them

Order matters, because the first result often decides whether the rest are needed. The Endocrine Society guideline on functional hypothalamic amenorrhea (periods stopping because the brain has turned the signal down) lists the core panel doctors run for absent or rare periods: hCG, TSH and free T4, prolactin, FSH, LH, estradiol and AMH (anti-Müllerian hormone, a marker of how many egg follicles remain), with total testosterone added when there are signs of androgen excess ³. From the PCOS page: "No single blood test shows PCOS." The extra tests in step 3 confirm high androgens and rule out look-alikes.

1. 1Rule out pregnancyA blood or urine hCG test, even if you think pregnancy is unlikely. If it is positive, the hormone panel below no longer applies; see a doctor about the pregnancy first.
2. 2The first-line panelTSH (with free T4 if TSH is off), prolactin, FSH, LH and estradiol. Together these show whether the thyroid, the pituitary, the brain signal or the ovaries are the problem.
3. 3If PCOS is on the tableTotal testosterone (high testosterone), SHBG to work out free testosterone, DHEA-S (an androgen made mainly by the adrenal glands; see high DHEA-S), 17-hydroxyprogesterone to exclude non-classic CAH, and, in adults, AMH. Add a glucose check (the 2023 guideline prefers a 75 g oral glucose tolerance test; fasting glucose or A1C if not) and a lipid panel, because PCOS travels with insulin resistance ².
4. 4Check whether you ovulateA progesterone blood test about 7 days before the next expected period; a clearly raised result means ovulation happened that cycle (a low one is covered on low progesterone). Urine LH strips at home show the surge that comes just before ovulation.

CalculatorFree testosterone calculatorHave total testosterone and SHBG? This estimates the free fraction that drives acne and hair growth.Work it out

What the results usually look like, cause by cause

Read LH, FSH and estradiol together: each cause moves the three in its own direction. The table summarises the typical patterns from the PCOS, functional hypothalamic amenorrhea, premature ovarian insufficiency and STRAW+10 guidance ^{2 3 5 7}; a doctor reads them alongside symptoms and an ultrasound where needed.

For premature ovarian insufficiency, the 2024 guideline from the European Society of Human Reproduction and Embryology (ESHRE) accepts one FSH above 25 IU/L (international units per litre, the usual lab unit) in someone under 40 with at least 4 months of rare or absent periods ⁷. Hypothalamic amenorrhea, by contrast, is a diagnosis of exclusion: the Endocrine Society guideline asks doctors to rule out the other causes before settling on it ³. The thyroid panel guide covers TSH and free T4 in more depth.

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When to see a doctor

• No period for 3 months if your cycles were regular, or 6 months if they were already irregular ¹¹
• Cycles shorter than 21 days or longer than 35 days (21 to 45 days in the first years after periods start), or fewer than 8 periods a year ^{2 4}
• Very heavy periods, or signs of anemia such as tiredness, breathlessness on stairs or pale skin (a ferritin result shows whether iron stores are running low)
• Acne, extra facial or body hair, or weight gain alongside irregular cycles
• Milky nipple discharge when not breastfeeding
• No period by age 15, or within 3 years of breast development
• Trying to conceive with irregular cycles

Why it's worth sorting out, even if you don't want a period

Fewer periods can feel like a relief, but long gaps without ovulation carry costs of their own. Without ovulation, the ovary makes no progesterone, so the womb lining keeps thickening under estrogen alone. The 2023 PCOS guideline flags the resulting rise in endometrial cancer risk and asks clinicians to keep it in mind, especially with long gaps between periods ². When irregular periods come from low estrogen, as in hypothalamic amenorrhea, bone density falls, raising the risk of osteoporosis; the Endocrine Society advises a DXA bone scan after 6 or more months without periods ³.

That cohort of about 80,000 nurses found the excess risk was largest for deaths from cardiovascular disease, and partly tracked with conditions such as type 2 diabetes that run alongside irregular cycles ⁶. Earlier work in the same cohort found that women whose cycles were long or very irregular at 18 to 22 had about double the later risk of type 2 diabetes ⁹. These are links, not proof of cause, so years of irregular cycles are a good reason to ask a doctor about checking blood sugar and lipids.

What helps, by cause

Treatment follows the cause, which is why the tests come first. What doctors usually offer, by cause:

• Stress or under-fuelling: eating enough to cover training is the main treatment. The Endocrine Society guideline puts restoring energy intake ahead of any medicine ³. Cutting training load and treating disordered eating where present come next.
• PCOS: losing 5 to 10% of body weight, where weight is above the healthy range, improves symptoms and can bring back ovulation ². Doctors may also offer the combined pill, metformin, or a progestogen (a progesterone-like hormone) taken for some days each month to protect the womb lining; these are prescription decisions made with your doctor.
• Thyroid disease and high prolactin: treating the thyroid or the prolactin source usually restores cycles ¹⁰. If a medicine is raising prolactin, ask your prescriber before changing it.
• Premature ovarian insufficiency: guidelines advise hormone replacement until the usual age of menopause, mainly to protect bone and heart, unless there is a reason not to ⁷; whether and how to take it is a decision to make with your doctor.
• After the pill, after pregnancy and while breastfeeding: cycles usually return without treatment; check in with a doctor if they have not by 6 months.
• Perimenopause: irregular cycles are expected. See a doctor for very heavy bleeding, bleeding between periods or periods closer than 21 days.

Tracking helps whatever the cause. Log the first day of every bleed for at least 3 cycles so you and your doctor can see the real spread. Calendar apps assume a regular cycle, so confirm ovulation as in step 4 above.

CalculatorOvulation calculatorGives a calendar estimate of your fertile window. With cycles that vary by more than a week, treat it as a rough guide only.Work it out

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