What Causes Irregular Periods? Common Reasons Explained

What Are Irregular Periods?

An irregular period means your menstrual cycle doesn't follow a predictable pattern. Your periods may arrive earlier or later than usual, happen at different intervals, or change in flow and duration.

A menstrual cycle doesn't have to be exactly 28 days to be healthy. According to the NHS, cycles between 21 and 35 days can be normal, although individual patterns vary.

Occasional changes aren't necessarily a sign of a health problem. However, if your normal pattern suddenly changes or your periods repeatedly become irregular, it's worth discussing with a healthcare professional.

What Causes Irregular Periods?

There are many possible causes of irregular periods. Some are temporary and harmless, while others may require medical evaluation or treatment.

Here are some of the most common reasons.

1. Puberty

Irregular periods are common during the first few years after menstruation begins.

The hormonal system that controls ovulation and menstruation is still developing, so periods may come earlier, later, or occasionally be skipped.

For many teenagers, cycles gradually become more predictable with time.

However, extremely long gaps between periods or other concerning symptoms should still be discussed with a healthcare professional.

2. Perimenopause

As menopause approaches, hormone levels naturally begin to change.

This transition is called perimenopause and commonly occurs during the 40s, although the timing varies.

During this stage, periods may become:

• Earlier or later

• Heavier or lighter

• Shorter or longer

• More or less frequent

The NHS notes that irregular periods are more common when approaching menopause, usually between ages 45 and 55.

3. Pregnancy

If you're sexually active and your period is late or missing, pregnancy is one of the first possibilities to consider.

A missed period can be an early sign of pregnancy.

If pregnancy is possible, taking an appropriate pregnancy test can help determine the next step.

4. Stress

Stress can affect the hormones involved in ovulation and menstruation.

Significant or ongoing stress may cause your period to arrive late, become irregular, or temporarily stop.

Stress-related menstrual changes may improve when the underlying stress decreases, although persistent irregular periods should still be evaluated.

5. Major Weight Changes

Significant weight loss or weight gain can affect menstrual regularity.

Very low body weight can interfere with hormonal function and ovulation, while being overweight can also be associated with menstrual irregularities.

Rapid changes are particularly important to mention to a healthcare professional if your periods change at the same time.

6. Too Much Exercise

Regular physical activity is generally beneficial, but very intense exercise combined with inadequate energy intake can interfere with normal reproductive hormone function.

People involved in intensive athletic training may experience irregular or absent periods, particularly when high energy expenditure is combined with low body fat or insufficient nutrition.

If your periods become irregular after significantly increasing your exercise routine, discuss the change with a healthcare professional.

7. Polycystic Ovary Syndrome (PCOS)

Polycystic ovary syndrome (PCOS) is a common hormonal condition that can cause irregular periods.

People with PCOS may ovulate less regularly, which can result in periods that are infrequent or unpredictable.

Other possible features can include:

• Excess facial or body hair

• Acne or oily skin

• Weight changes

• Difficulty becoming pregnant

Not everyone with PCOS has all of these symptoms. If irregular periods occur alongside other hormonal symptoms, a healthcare professional can assess the possible causes.

8. Thyroid Problems

Your thyroid produces hormones that affect many processes throughout the body, including the menstrual cycle.

Both an underactive thyroid (hypothyroidism) and an overactive thyroid (hyperthyroidism) can contribute to menstrual irregularities.

Other symptoms may include changes in weight, tiredness, changes in heart rate, temperature sensitivity, or changes in hair and skin.

If you have irregular periods along with these symptoms, your healthcare professional may consider thyroid testing.

9. Hormonal Birth Control

Hormonal contraception can change your usual menstrual pattern.

Depending on the method, you may experience:

• Lighter periods

• Irregular bleeding

• Spotting

• Fewer periods

• No periods

The contraceptive pill, injection, implant, and some intrauterine systems can affect menstrual bleeding patterns.

These changes aren't necessarily harmful, but new or unusual bleeding should be discussed with a healthcare professional when you're concerned.

10. Breastfeeding

Breastfeeding can delay the return of regular menstrual cycles after pregnancy.

Some people may not have periods for a while, while others experience irregular periods as menstruation returns.

The timing varies between individuals and can depend on factors such as how frequently breastfeeding occurs.

11. Uterine Fibroids

Uterine fibroids are noncancerous growths in or around the uterus.

They can cause changes in menstrual bleeding, including:

• Heavy periods

• Longer periods

• Bleeding between periods

• Pelvic pressure or discomfort

Fibroids are one possible cause of abnormal menstrual bleeding, particularly in adults of reproductive age.

12. Endometriosis

Endometriosis occurs when tissue similar to the lining of the uterus grows outside the uterus.

It can cause painful periods and may also be associated with heavy or irregular menstrual bleeding.

Other symptoms can include pelvic pain and pain during or after sex.

If period changes are accompanied by significant or persistent pelvic pain, medical evaluation is recommended.

13. Certain Medications

Some medications can affect menstrual cycles.

Mayo Clinic notes that certain medicines, including some antidepressants, antipsychotics, blood-pressure medications, allergy medicines, and chemotherapy drugs, may affect menstruation.

Don't stop prescribed medication because of menstrual changes without first speaking with the healthcare professional who prescribed it.

14. Other Health Conditions

Irregular periods can sometimes be associated with other medical conditions.

Depending on your symptoms and medical history, healthcare professionals may consider conditions affecting hormones, the reproductive organs, metabolism, or other body systems.

For this reason, persistent or newly developed irregular periods shouldn't always be dismissed as "just hormones."

When Should You Worry About Irregular Periods?

Irregular periods are not always a cause for concern, but certain changes deserve medical attention.

Consider speaking with a healthcare professional if:

• Your periods suddenly become irregular after being regular.

• Your periods repeatedly occur less than 21 days apart or more than 35 days apart.

• Your periods last longer than 7 days.

• You have bleeding between periods.

• You have very heavy menstrual bleeding.

• You have severe or worsening pelvic pain.

• You have irregular periods and are having difficulty becoming pregnant.

• You have missed three periods in a row and aren't pregnant.

• You experience irregular periods together with symptoms such as significant weight changes, unusual hair growth, or persistent tiredness.

If you experience extremely heavy bleeding, severe pain, fainting, or other urgent symptoms, seek prompt medical care.

How Can You Track Irregular Periods?

Tracking your menstrual cycle can make it easier to identify changes.

Record:

• The first day of each period

• The last day of bleeding

• How heavy the bleeding is

• Spotting between periods

• Pain or cramps

• Other symptoms

• Major changes in stress, weight, exercise, or medication

You can use a calendar, diary, or period-tracking app.

The first day of bleeding is generally considered day 1 of the menstrual cycle.

Tracking several cycles can give you and your healthcare professional a clearer picture of your usual pattern.

How Are Irregular Periods Diagnosed?

The evaluation depends on your symptoms and medical history.

A healthcare professional may ask about:

• Your menstrual history

• Pregnancy possibility

• Medications and contraception

• Weight changes

• Exercise

• Stress

• Other symptoms

• Your reproductive and medical history

Depending on the situation, they may recommend pregnancy testing, blood tests, imaging, or other examinations to identify the underlying cause.

Treatment isn't always necessary. The appropriate treatment depends on what's causing the irregular periods.

Can Irregular Periods Affect Pregnancy?

Sometimes.

If irregular periods are caused by irregular ovulation, predicting when ovulation occurs can become more difficult.

Some conditions associated with irregular periods, such as PCOS, can also make becoming pregnant more difficult for some people.

However, having irregular periods doesn't automatically mean you cannot become pregnant.

If you're trying to conceive and your periods are consistently irregular, consider speaking with a healthcare professional.

The Bottom Line

What causes irregular periods? There are many possible reasons, including puberty, perimenopause, pregnancy, stress, major weight changes, intense exercise, hormonal contraception, PCOS, thyroid problems, breastfeeding, fibroids, endometriosis, and certain medications.

Occasional changes may be normal, but a persistent or sudden change in your menstrual pattern deserves attention.

Tracking your periods and discussing significant changes with a healthcare professional can help identify the cause and determine whether treatment is needed.

Frequently Asked Questions

What is the most common cause of irregular periods?

There isn't one single cause for everyone. Common causes include puberty, pregnancy, stress, weight changes, hormonal contraception, perimenopause, PCOS, and thyroid problems.

Can stress cause irregular periods?

Yes. Significant stress can temporarily interfere with the hormonal signals that regulate ovulation and menstruation, potentially causing periods to become late, irregular, or absent.

Can weight gain or weight loss affect periods?

Yes. Significant weight changes can affect hormonal function and menstrual regularity.

Can PCOS cause irregular periods?

Yes. PCOS can interfere with regular ovulation and commonly causes irregular or unpredictable periods.

Can thyroid problems cause irregular periods?

Yes. Both an underactive and overactive thyroid can contribute to menstrual irregularities.

How long is too long without a period?

If you've missed three periods in a row and you're not pregnant, the NHS recommends speaking with a healthcare professional.

Are irregular periods a sign of infertility?

Not necessarily. However, irregular periods can sometimes indicate irregular ovulation, which may make pregnancy more difficult. If you're trying to conceive, discuss persistent irregular cycles with a healthcare professional.

Can irregular periods be treated?

Often, yes, but treatment depends on the underlying cause. Some people don't need treatment, while others may benefit from treatment for an underlying hormonal or reproductive condition.

Reliable Health References

• NHS — Irregular Periods — Causes, symptoms, when to seek medical advice, and treatment information.

• NHS — Period Problems — Information about irregular periods, endometriosis, menstrual bleeding, and other period concerns.

• Mayo Clinic — Menstrual Cycle: What's Normal, What's Not — Information about normal menstrual cycles and causes of irregularities.

• Mayo Clinic — Amenorrhea: Symptoms and Causes — Causes of missed or absent periods, including pregnancy, stress, exercise, weight changes, PCOS, thyroid conditions, and medications.