Irregular Periods: Why Your Cycle Changes

What counts as an irregular period

A normal menstrual cycle, counted from the first day of one period to the first day of the next, is 24 to 38 days in adults. Bleeding usually lasts up to 8 days. Most people's cycles vary by a few days, which is normal. Doctors consider cycles irregular when:

• they vary by more than 7 to 9 days from one cycle to the next
• they are consistently longer than 38 days (infrequent periods, oligomenorrhea)
• they are shorter than 24 days (frequent periods)
• periods stop for 3 months or more (amenorrhea)

In teenagers, cycles of 21 to 45 days are normal in the first few years.

Times when irregular periods are expected

• Puberty: ovulation is often irregular for 2 to 3 years after the first period.
• After pregnancy and while breastfeeding: periods may be absent or irregular for months.
• After stopping hormonal contraception: it can take up to 3 months for cycles to settle.
• Perimenopause: in the 4 to 10 years before menopause, cycles often become shorter and then longer, with skipped periods. See perimenopause.

Common causes

Pregnancy

A late or missed period is often the first sign. Home pregnancy tests are reliable from the day the period was due. Early pregnancy bleeding can be mistaken for a light period.

Polycystic ovary syndrome (PCOS)

PCOS is the most common hormonal cause of irregular periods, affecting roughly 1 in 10 women of reproductive age. Typical signs are infrequent or absent periods, acne, excess hair on the face or body, scalp hair thinning and weight gain. It is linked to insulin resistance and is a common cause of difficulty getting pregnant.

Stress, weight and exercise

The brain's hypothalamus controls the cycle and is sensitive to energy balance and stress. Hypothalamic amenorrhea occurs with:

• intense exercise, especially in endurance athletes and dancers
• low body weight, dieting or eating disorders
• severe emotional stress, illness or travel across time zones

Significant weight gain can also disrupt ovulation.

Thyroid problems

Both an underactive thyroid (hypothyroidism) and an overactive thyroid can make periods irregular. Hypothyroidism often makes them heavier, while hyperthyroidism can make them lighter and less frequent.

High prolactin

Hyperprolactinemia, from a small pituitary tumor or medicines such as some antipsychotics, stops ovulation and can cause milky nipple discharge.

Contraception and medicines

Progestin-only pills, implants, injections and hormonal IUDs often cause irregular spotting or stop periods altogether. Missed combined pills cause breakthrough bleeding. Other medicines that can disrupt cycles include antipsychotics, some antidepressants, steroids and chemotherapy.

Uterine problems

Fibroids, polyps and adenomyosis usually cause heavy or prolonged periods and bleeding between periods rather than skipped cycles.

Premature ovarian insufficiency

Premature ovarian insufficiency is loss of normal ovarian function before 40. Periods become irregular then stop, often with hot flashes and vaginal dryness.

Clues that point to the cause

How doctors find the cause

A doctor will ask about your cycle pattern (a period-tracking app or diary helps), contraception, weight changes, exercise, stress, medicines, hair growth and acne, and pregnancy plans. Examination includes weight, blood pressure, skin and hair, and sometimes a pelvic exam.

Tests often include:

• pregnancy test
• thyroid function (TSH) and prolactin
• FSH and LH, estradiol, and testosterone if PCOS is suspected
• blood count if bleeding is heavy
• pelvic ultrasound to look at the ovaries and lining of the womb

Treatment

Treatment depends on the cause and your goals:

• Lifestyle: regaining weight or energy balance restores periods in hypothalamic amenorrhea. In PCOS, losing 5% to 10% of body weight can restart ovulation.
• Hormonal contraception: the combined pill, patch or ring regulates bleeding and protects the womb lining.
• Cyclical progestogen: triggers a regular bleed in people who do not want hormonal contraception.
• Treating thyroid or prolactin problems usually restores cycles.
• Fertility treatment: ovulation-inducing medicines such as letrozole for PCOS if you want to conceive.

Why infrequent periods matter

Going many months without a period, especially with PCOS, lets the womb lining build up without shedding. Over years this raises the risk of endometrial hyperplasia and cancer. Doctors usually recommend at least 4 bleeds a year, or a hormonal treatment that protects the lining. Long-term low estrogen, as in hypothalamic amenorrhea, also thins the bones.

See the irregular menstruation condition page for more on diagnosis, long-term health effects and treatment options.

Frequently asked questions

What causes irregular periods?

The most common causes are pregnancy, stress, weight change, intense exercise, PCOS, thyroid disease, hormonal contraception and breastfeeding. Irregular periods are also normal for a few years after periods start and in the years leading up to menopause.

Can you be pregnant with irregular periods?

Yes. Irregular periods make it harder to know when you ovulate, but ovulation still happens in many cycles. If you have had sex without contraception and your period is late, take a pregnancy test.

How late can a period be before I should worry?

A period that is up to a week late after a stressful month is usually not a concern if a pregnancy test is negative. See a doctor if you miss 3 periods in a row, your cycles are regularly longer than 38 days, or you have other symptoms such as excess hair growth.

Are irregular periods a sign of PCOS?

They can be. Infrequent or absent periods are one of the main features of PCOS, especially with acne, excess hair growth and weight gain. But many other conditions also cause irregular periods, so blood tests and sometimes an ultrasound are needed to confirm it.

Can stress make your period irregular?

Yes. Physical or emotional stress can suppress the hormones from the brain that trigger ovulation, delaying or skipping a period. Cycles usually return to normal once the stress eases. Persistent changes should be checked for other causes.