Unpredictable Menstruation (Irregular Periods)

What counts as an irregular period

Your cycle is counted from the first day of one period to the first day of the next. Tracking for 3 months in an app or calendar makes patterns easier to see. Bleeding between periods is covered in intermenstrual bleeding.

Normal life stages with irregular periods

The teen years

In the first 1 to 3 years after periods start, ovulation is often irregular, so cycles can range from 21 to 45 days. Most settle by 3 years. Cycles that remain very irregular, or periods that are very heavy, should be checked.

Perimenopause

In the 40s, ovulation becomes unpredictable. Periods can come closer together, then further apart, with skipped months, heavier or lighter flow, hot flashes and sleep changes. Perimenopause lasts about 4 to 8 years before the final period.

After pregnancy and while breastfeeding

Breastfeeding suppresses ovulation, so periods may be absent or irregular for months.

Common medical causes

Polycystic ovary syndrome (PCOS)

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

Stress, weight and exercise

The brain's hypothalamus controls the cycle. Physical or emotional stress, low body weight, rapid weight loss, eating disorders and intense training can switch off ovulation (hypothalamic amenorrhea). Significant weight gain can also disrupt cycles.

Thyroid problems

Both an underactive thyroid (hypothyroidism) and an overactive thyroid can cause irregular, heavy or absent periods, along with tiredness, weight change and temperature intolerance.

High prolactin

Hyperprolactinemia, often from a small benign pituitary tumor or certain medicines (some antipsychotics and anti-nausea drugs), causes irregular or absent periods and sometimes milky nipple discharge.

Contraception and medicines

Hormonal birth control commonly causes irregular bleeding, especially progestogen-only methods (implant, injection, mini-pill, hormonal IUD). Missed pills, emergency contraception, and stopping the pill can also temporarily disrupt cycles. Other medicines include antipsychotics, some antidepressants, steroids and chemotherapy.

Uterine and ovarian conditions

• Fibroids, polyps and adenomyosis (usually cause heavy or intermenstrual bleeding)
• Primary ovarian insufficiency: loss of ovarian function before age 40, with irregular or absent periods and menopausal symptoms
• Endometrial hyperplasia, often linked to long gaps between periods

Other causes

• Pregnancy and miscarriage
• Uncontrolled diabetes, celiac disease, and chronic kidney or liver disease
• Adrenal conditions, such as non-classic congenital adrenal hyperplasia and Cushing's syndrome

Why irregular periods matter

Occasional irregularity is not harmful. Long-term infrequent periods can lead to:

• Endometrial hyperplasia: without regular ovulation, the womb lining is exposed to estrogen without progesterone and can thicken. Over years this raises the risk of endometrial hyperplasia and womb cancer. Doctors usually advise having at least 4 bleeds a year, naturally or with hormones.
• Difficulty conceiving, because ovulation is unpredictable or absent.
• Bone thinning, when low estrogen is the cause (for example, from low weight or primary ovarian insufficiency).

How doctors find the cause

A doctor will ask about your cycle pattern, contraception, weight changes, exercise, stress, hair growth, acne, nipple discharge, headaches and plans for pregnancy. Tests often include:

• pregnancy test
• blood tests: TSH (thyroid), prolactin, FSH and LH, estradiol, testosterone, and sometimes AMH, glucose and cholesterol
• pelvic ultrasound to look at the ovaries and womb lining
• endometrial biopsy if you are over 45 or have risk factors and persistent irregular bleeding

Treatment

Treatment depends on the cause and your goals:

• Lifestyle: reaching a healthy weight, eating enough for your activity level, and managing stress can restore regular cycles. In PCOS, losing 5 to 10 percent of body weight often improves ovulation.
• Hormonal contraception: the pill, patch, ring or hormonal IUD regulates bleeding and protects the womb lining.
• Cyclic progestogen: a progestogen taken for 10 to 14 days every 1 to 3 months to trigger a regular bleed.
• Specific treatments: thyroid medicine, prolactin-lowering medicine, or metformin for insulin resistance in PCOS.
• Fertility treatment: ovulation-inducing medicines such as letrozole for people trying to conceive.
• Hormone therapy: for primary ovarian insufficiency, to protect bones and heart.

Frequently asked questions

Why are my periods so unpredictable?

The most common reasons are being in the first years after periods start or approaching menopause, PCOS, stress, weight changes, intense exercise, thyroid problems and hormonal contraception. If your periods have become irregular and you are not pregnant, ask your doctor for a check.

Is it normal to have irregular periods?

It is normal in the first 2 to 3 years after your first period and in the years leading up to menopause. An occasional early or late period at other times is also common. Persistent irregular cycles in adulthood usually have a cause worth finding.

Can stress cause irregular periods?

Yes. Physical or emotional stress can suppress the hormones that trigger ovulation, delaying or stopping periods. Cycles usually return to normal once the stress eases. Persistent irregularity should still be checked for other causes.

When should I see a doctor about irregular periods?

See a doctor if you miss 3 periods in a row without being pregnant, your cycles are consistently shorter than 24 or longer than 38 days, you bleed between periods or after sex, or you have excess hair growth, acne or nipple discharge.

Can you get pregnant with irregular periods?

Yes, but it can be harder because ovulation is less predictable or happens less often. Ovulation tests and tracking can help. If you are trying to conceive and your periods are irregular, see a doctor early rather than waiting a year.