Irregular Menstrual Periods: Causes and When to See a Doctor

What is a normal menstrual cycle?

A cycle is counted from the first day of one period to the first day of the next. For adults, a normal cycle is 24 to 38 days, with bleeding lasting up to 8 days. Few people have exactly 28 days every month, and a variation of up to 7 to 9 days between your shortest and longest cycles over a year is normal. In teenagers, cycles of 21 to 45 days are normal in the first few years.

Irregular periods can mean:

• cycles that vary unpredictably in length
• infrequent periods (oligomenorrhea), more than 38 days apart
• frequent periods, less than 24 days apart
• missed periods for 3 months or more (amenorrhea)
• bleeding or spotting between periods

Common causes of irregular periods

Pregnancy

Always consider pregnancy first, even if you use contraception. Take a home pregnancy test if a period is late.

Polycystic ovary syndrome (PCOS)

PCOS affects about 1 in 10 women of reproductive age and is the most common hormonal cause of irregular periods. The ovaries make more androgens (male-type hormones) and do not release an egg regularly. Clues include infrequent periods, acne, excess facial or body hair, scalp hair thinning, weight gain and difficulty getting pregnant.

Stress, weight and exercise

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

• significant weight loss or very low body weight
• intense training, especially in running, dance and gymnastics
• eating disorders
• severe emotional stress or illness

Significant weight gain can also disrupt ovulation.

Hormonal contraception

The pill, patch, ring, implant, injection and hormonal IUD often cause irregular bleeding, spotting or no periods. This is usually harmless. Irregular bleeding in the first 3 months of a new method is especially common. Periods can take a few months to settle after stopping hormonal contraception, particularly the injection.

Perimenopause

In the years before menopause, usually from the mid-40s, cycles become shorter, then longer and unpredictable, with skipped periods. See perimenopause.

Breastfeeding

Breastfeeding suppresses ovulation, so periods may not return or may be irregular for months.

Medical causes

• Thyroid disorders: both an underactive thyroid (hypothyroidism) and an overactive thyroid disrupt cycles.
• High prolactin: from a small pituitary tumor or medicines such as some antipsychotics. Can cause missed periods and milky nipple discharge.
• Primary ovarian insufficiency: the ovaries stop working normally before age 40, with hot flashes and missed periods.
• Structural causes: polyps, fibroids and adenomyosis usually cause heavy or between-period bleeding rather than irregular cycle length.
• Uncontrolled diabetes, celiac disease, kidney or liver disease.
• Medicines: antipsychotics, some antidepressants, epilepsy medicines, steroids and chemotherapy.
• Endometrial hyperplasia or cancer: rare in young women, but more likely with long-term irregular periods, obesity or PCOS, and in people over 45 with irregular bleeding.

Irregular periods in teenagers

In the first 2 to 3 years after periods start, irregular cycles are normal because ovulation is not yet regular. See a doctor if periods have not started by 15, if a teen goes more than 90 days without a period, if cycles are shorter than 21 days, or if bleeding is very heavy or lasts more than 7 days. Teen PCOS and eating disorders should be considered.

When to worry about irregular periods

See a doctor if you:

• have missed 3 periods in a row and are not pregnant
• have cycles shorter than 21 days or longer than 45 days
• bleed between periods or after sex
• have very heavy periods or periods lasting more than 8 days
• have signs of PCOS, thyroid disease or high prolactin
• are trying to get pregnant and periods are irregular
• have any bleeding after menopause

Long gaps between periods are not harmless in the long term: without regular shedding, the womb lining can thicken, which raises the risk of endometrial hyperplasia and cancer. Treatment usually ensures at least four bleeds a year.

How doctors find the cause

Your doctor will ask about your cycle pattern (a period-tracking app helps), contraception, weight change, exercise, stress, medicines, hair growth, acne and pregnancy plans. Tests may include:

• pregnancy test
• blood tests: thyroid function (TSH), prolactin, FSH and LH, estradiol, testosterone, and sometimes AMH; blood sugar and lipids if PCOS is suspected
• pelvic ultrasound: to check the ovaries, the womb lining, fibroids and polyps
• endometrial biopsy: for some women over 45, or younger with risk factors, who have irregular bleeding

Treatment

Treatment depends on the cause and on whether you want to get pregnant.

• PCOS: lifestyle changes (even 5% weight loss can restore ovulation), combined hormonal contraception to regulate cycles and reduce acne and hair growth, metformin in some cases, and ovulation induction for fertility.
• Thyroid disease: thyroid hormone replacement or antithyroid treatment usually restores normal cycles.
• High prolactin: medicines that lower prolactin.
• Hypothalamic amenorrhea: restore energy balance with increased nutrition and reduced training, often with dietitian and mental health support.
• Perimenopause: hormonal contraception, the hormonal IUD or menopausal hormone therapy can control erratic bleeding and symptoms.
• Protecting the womb lining: a progestogen course, the pill or a hormonal IUD for those with infrequent periods.

Frequently asked questions

Why are my periods irregular?

The most common reasons are pregnancy, polycystic ovary syndrome, stress, significant weight change, intense exercise, thyroid problems, hormonal contraception, breastfeeding and perimenopause. In teenagers, irregular periods in the first few years are normal. A doctor can test for the cause if irregularity persists.

When should I worry about irregular periods?

See a doctor if you miss 3 periods in a row without being pregnant, have cycles shorter than 21 or longer than 45 days, bleed between periods or after sex, have very heavy bleeding, or have signs of PCOS. Any bleeding after menopause needs prompt assessment.

Can stress cause irregular periods?

Yes. Severe or prolonged stress affects the hypothalamus in the brain, which controls the hormones that trigger ovulation. Periods may come late, be lighter, or stop for a while. They usually return to normal when stress eases.

Can I get pregnant with irregular periods?

Yes, but it may take longer, because you may ovulate less often and it is harder to predict fertile days. Ovulation predictor kits can help. If you are under 35 and have not conceived after 12 months, or sooner if periods are very irregular, see a doctor.

How can I regulate my periods naturally?

Maintaining a healthy weight, eating enough to fuel your activity, managing stress and getting enough sleep can help cycles become more regular. In PCOS, modest weight loss often restores ovulation. If periods remain irregular, see a doctor, as treatment may be needed.