Why your teen has an irregular period

A guide to late periods in teens

Quick summary

• Irregular or late periods are common in the first few years after menstruation begins
• Factors like stress, hormones, or weight changes can all affect the menstrual cycle
• Speak to a doctor if your preteen or teen’s period is very irregular after 2 years or they’ve missed more than 3 periods in a row

Rated 4.8

Empower your teen today. Subscribe them to luna.

Find out more

Worried your preteen or teen’s period is irregular or running late? You’re not alone.

Research shows that around 41% of girls report irregular menstrual cycles – with younger teens more likely to be affected than older teens.

In the early years after periods begin (typically between ages 9-14), irregularity is very common and often nothing to worry about.

What does it mean if a girl has irregular periods?

Periods are considered irregular if:

• The cycle length (from one period to the next) keeps changing
• They come too often (e.g. every 2 weeks) or not often enough
• Periods last longer than 7 days, or shorter than 2 days, or vary a lot in flow

A typical cycle is 21-35 days, and periods usually last 2–7 days. But during puberty, these ranges can vary a lot.

What counts as irregular periods?

A "regular" period doesn’t mean it'll come on the exact same date every month.

Periods are part of a monthly menstrual cycle, which is the time from the start of one period to the start of the next.

Most menstrual cycles are:

• 21 to 35 days long
• With 2 to 7 days for the period itself

Since months are usually 30-31 days, the dates of a "regular period" can shift each month.

A regular period means it arrives and lasts in a way that’s consistent for your teen.

So what counts as an irregular period is simply that it is not normal or consistent for what your teen is used to.

Why does my daughter have irregular periods?

The menstrual cycle is controlled by hormones – and during puberty, those hormones are still regulating.

Common causes of irregular periods or late periods include:

• Puberty: it can take around 2 years from a first period for cycles to settle
• Stress or anxiety: emotional stress can delay or interrupt cycles
• Weight changes or restrictive eating: losing or gaining weight quickly affects hormone levels
• High levels of physical activity: especially in sports like gymnastics, dance, or swimming
• Illness or change in routine: sickness or disrupted sleep can interfere with hormones

These are all common and usually temporary. In many cases, irregular cycles will become more consistent over time.

What counts as abnormal menstruation?

There are different types of irregularities that may need medical attention, such as:

• Regularly bleeding between periods
• Missing a period for 3 or more cycles in a row 
• Periods regularly lasting less than 2 days or more than 7 days
• Very heavy periods (for example needing to change a pad or tampon every 1-2 hours or more frequently) or periods associated with large clots (larger than the size of a 10p coin)
• Periods regularly occurring close together (for example cycles consistently shorter than 21 days)

Is it normal for my daughter to have irregular periods?

Yes. 

Having irregular periods – including skipping a few months – is very common for all teenagers, whether that’s a 12 year old or a 15 year old.

It typically takes a couple of years after a first period for cycles to become more regular. 

So it really depends on how long your teen has been experiencing periods as to whether that it is or isn’t normal.

When should I be concerned about my daughter's period?

It’s important to see a doctor if:

• Your teen has not had a period by age 15
• They’ve missed more than 3 periods in a row
• Their periods are still very irregular more than 2 years after their first
• They are bleeding heavily or for more than 7 days
• They are experience severe pain, feel dizzy, or are very tired during periods
• They experience bleeding between periods

What if my teenager has missed period for 3 months in a row?

In this case, it’s best to see a doctor – just as a precaution.

Are there any effects of irregular periods on the body?

Irregular periods on their own are not usually harmful. 

But in some cases, they may be a symptom of underlying issues such as:

• Polycystic ovary syndrome (PCOS)
• Thyroid conditions
• Hormonal imbalances
• Disordered eating or undernutrition

It’s a good idea to encourage your daughter to track symptoms using luna’s period tracker, and get medical advice if they’re experiencing other changes in their health or energy.

Rated 4.8

Empower your teen today. Subscribe them to luna.

Find out more

How to cure irregular periods in teenage girls?

Unfortunately there’s no single treatment for irregular periods, but supporting your teen’s general health can help regulate their cycle:

• Make sure they’re getting balanced meals and staying hydrated 
• Help them manage stress
• Support good sleep habits
• Encourage them to track their cycle using an app like luna
• Reassure them that they can always ask for help if they have any concerns about their periods or general health

Rated 4.8

Empower your teen today. Subscribe them to luna.

Find out more

Irregular periods are usually part of growing up – especially in the first few years after starting menstruation.

But if your daughter is missing periods for long stretches, bleeding heavily, or feeling unwell, it’s always okay to speak to a doctor.

Tracking patterns on luna and keeping communication open can make a big difference in helping them feel supported and confident about their body.