In short

If your sporty daughter has lost her period, it's often a sign she's not eating enough to match what her training, growing, and everyday life ask of her body. 

Sports medicine calls this Relative Energy Deficiency in Sport (RED-S). It's common in active teen girls, treatable, and worth seeing your GP about. Untreated, it can affect her bones, hormones, and mental health.

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Why has my sporty daughter's period stopped?

Most of the time, it's because her body isn't getting enough fuel to keep everything running. When energy is low, the body treats periods as non-essential and switches them off to protect more critical systems.

Common causes of missing or irregular periods in active teens are losing weight, exercising a lot, and stress. In girls who train hard, these often overlap. 

A growing teenager who's also doing competitive sport needs a lot more food than most people realise.

Other things worth ruling out with a GP include pregnancy, hormonal contraception, polyendocrine metabolic ovarian syndrome (PMOS), and an underactive thyroid.

What is RED-S?

RED-S stands for Relative Energy Deficiency in Sport. It's the medical term for what happens when a teen is exercising more than her body has fuel for. 

You may have heard the older term "female athlete triad". RED-S is the up to date name, and it applies to both girls and boys.

When her body senses low energy availability, it cuts back on the systems it doesn't need to survive: periods, bone-building, immune function, mood regulation, and growth. 

Short term, that shows up as missed periods, tiredness, and more injuries. Long term, it can lower her bone density and raise her risk of stress fractures.

RED-S is most common in sports that emphasise leanness or endurance, like dance, gymnastics, distance running, swimming, cycling, and rowing. But it can affect any active teen.

What are the signs of RED-S in my daughter?

The most obvious sign is a missing or irregular period. But there are usually other clues alongside.

Common signs to look for:

• Missed, irregular, or much lighter periods
• Tiredness that doesn't lift with rest
• More injuries than usual, especially stress fractures or shin splints
• Slower recovery between training sessions
• Low mood, anxiety, or feeling flat
• Feeling cold a lot, even in warm rooms
• Performance plateauing or getting worse despite training harder
• Hair thinning or brittle nails

She doesn't need to look thin for RED-S to be the cause. It can affect teens at any size. What matters is the gap between what she's eating and what her body needs.

Is it normal for athletic teens to skip periods?

It can be common, but it isn't normal. A missing period in a teen athlete is a red flag, not a perk of training.

In a luna poll of 1,049 teen girls, 70% said their period had made them want to skip PE. That's one side of the picture: periods getting in the way of sport. 

The other side, where the sport itself stops the period, is a medical issue. 

How long should I wait before seeing a GP?

Book a GP appointment if your daughter has missed three periods in a row, if her periods have stopped for six months once they were established, or if her periods haven't started by age 15.

The NHS also recommends seeing a GP if her periods are irregular alongside symptoms like tiredness, weight changes, or hair growth on her face. 

You don't need to wait for more things to go wrong. The earlier RED-S is caught, the faster the recovery.

What your GP might do:

• Take a full history of her training, eating, periods, sleep, and mood
• Check her weight, height, blood pressure, and heart rate
• Run blood tests for hormones, thyroid, iron, and vitamin D
• Refer to a paediatric gynaecologist, endocrinologist, or sports medicine specialist if needed
• Recommend a bone density scan in some cases

How is RED-S treated?

Treatment is rarely about taking her off sport. It's about closing the gap between what she's consuming and what she's burning and focusing on healthy eating.

The core steps:

• Eat more, especially carbohydrates around training. This is the single biggest lever
• Reduce training load temporarily if her doctor advises
• Work with a sports dietitian who specialises in adolescents if you can access one
• Address any underlying disordered eating with a specialist
• Treat any bone or hormonal effects that are already showing up

Periods usually return within 6 to 12 months once energy availability is restored, though it can take longer. 

Bone health takes longer than that to rebuild, which is one of the reasons catching RED-S early really matters.

How do I talk to my daughter about this without making it about food?

This is the part most parents worry about, and rightly so. Teen girls are already swimming in messages about bodies, food, and shape.

A few things that help:

• Frame it as fuel, not weight. "Your body needs more petrol for the training you're doing" lands better than "you need to eat more"
• Make it about performance, energy, and avoiding injury, which are outcomes she cares about
• Avoid words like "diet", "calories", or "burn off", and steer clear of comments about her body shape
• Bring in a third party. A GP, sports physio, or registered dietitian carries weight that parents sometimes don't
• Watch your own language about food and bodies at home

In a luna poll of 2,053 teen girls, 1 in 3 (32%) said the best way parents could support them when stressed out was emotional support: listening without judgement or creating a calm environment. RED-S conversations are no different.

If you spot signs of disordered eating, that needs specialist help. RED-S and eating disorders overlap, but they aren't the same thing, and treating one without the other doesn't usually work.

Could her missing period be something else?

Yes, RED-S is one cause, not the only one. Other things to rule out with your GP include:

• Pregnancy
• Hormonal contraception, including the progestogen-only pill, the injection, and the hormonal coil
• Polyendocrine metabolic ovarian syndrome (PMOS)
• An underactive thyroid
• Stress and anxiety, which can also overlap with RED-S
• A natural settling-in phase. In a luna poll of 1,605 teen girls, only 13% said their periods were regular straight away

If her periods only started in the last year or two, some irregularity is expected. 

But a period that vanishes after being established, or one that's been missing for more than three months, deserves a doctor's look.

FAQ

Can my daughter keep training if she has RED-S?

In most cases yes, but at a reduced load and with more fuel. Her doctor or a sports medicine specialist will advise. 

The goal isn't punishment, it's getting her body what it needs while she keeps doing what she loves.

Does RED-S happen to boys too?

Yes. It used to be thought of as a girl's issue, but boys can be affected too. The signs are different because boys don't have a missed period as an early flag, so it's often spotted later.

Low energy, frequent injuries, low mood, and poor recovery are common signs.

Will my daughter's period come back?

Most girls' periods return within 6 to 12 months once they're eating enough to fuel their training. Bone health takes longer to recover, which is why early action matters.

Should I stop her doing sport?

Usually no. Sport is good for teens' bodies and mental health, and pulling it away often backfires. 

The fix is fuelling properly, not stopping. Her doctor can advise if a temporary reduction is sensible.

What if she resists eating more?

If she's resistant to eating more, especially around certain food groups, that often points to underlying disordered eating that needs specialist help. 

Your GP can refer you to a child and adolescent eating disorders service.

What should I do next?

If her period has stopped, you're already doing the right thing by paying attention. Book a GP appointment this week, keep meals regular at home, and remind her this isn't her fault. 

Her body is asking for more fuel, not less. Most teens recover their cycles once their fuelling matches their training, and so does the worry that's brought you here.

Alongside this support, it is helpful to watch out for disordered eating signs, such as calorie counting or if your daughter thinks she’s fat when she’s not.

Rated 4.8

Period tracking & more for teens. Guidance for parents.

Find out more