Prader-Willi syndrome can cause a wide range of symptoms, and affect your child's physical, psychological and behavioural development.


Floppiness caused by weak muscles is usually noticed shortly after birth. The medical name for this is hypotonia.

Hypotonia can mean your baby:

  • does not have a full range of movement
  • has a weak cry
  • has poor reflexes
  • cannot suck properly, which means they may have feeding difficulties and be underweight until the age of 1, and may need to be fed with a feeding tube (see managing Prader-Willi syndrome)

Poor sexual development

Baby boys may have an abnormally small penis, and one or both testicles may still be inside their abdomen (undescended testicles). Baby girls will also have underdeveloped genitalia.

Children with Prader-Willi syndrome experience puberty later than usual and may not go through full development into an adult. For example:

  • boys may still have a high-pitched voice, and may not have much facial and body hair
  • girls will often not start their periods until they're in the 30s, and their breasts will not fully develop – when periods do start, they're usually irregular and very light

It's almost unknown for either men or women with Prader-Willi syndrome to have children. They're usually infertile because the testicles and ovaries do not develop normally. But sexual activity is usually possible, particularly if sex hormones are replaced.

Distinctive features

Children with Prader-Willi syndrome have several distinctive features, including:

  • almond-shaped eyes
  • eye problems
  • a narrow forehead at the temples
  • narrow bridge of the nose
  • a thin upper lip and a downturned mouth
  • unusually fair hair, skin and eyes
  • small hands and feet

These features are usually noticed at or shortly after birth.

Increased appetite

Children with Prader-Willi syndrome develop an increased appetite and eat an excessive amount of food if they have the opportunity. The medical name for this is hyperphagia.

Between the ages of 1 and 4, the child will start to show an increased interest in food, ask for extra food and behave badly to get extra food. This behaviour increases during childhood and is seen in all children with Prader-Willi syndrome. It can include:

  • tantrums and angry behaviour to get food
  • interest in pictures of food, food-related play or discussion of food
  • hiding or stealing food, or stealing money to get food
  • eating inappropriate items, frozen or uncooked food, or waste food
  • very good memory for people who've given food to them in the past or situations where food was available

Children with Prader-Willi syndrome can tolerate much larger amounts of food before their body automatically vomits it back up, and they're not as sensitive to pain. Therefore, they can eat objects that would make other people very ill.

This means they're at higher risk of:

  • food poisoning
  • choking
  • a stomach rupture – where the stomach splits open, sometimes causing life-threatening infection

Vomiting and stomach pain are the 2 most common signs of a gastric rupture. People with Prader-Willi syndrome do not usually vomit or complain about stomach pain, so take these symptoms very seriously if they happen to your child. Call your care team or GP immediately for advice.

Children with Prader-Willi syndrome do not always become obese if their diet and food issues are controlled. They'll never learn to control their food intake themselves and it must be managed for them.

They also have a slow metabolism, and require less food and fewer calories than other children of the same age. Reduced muscle tone means they're less physically active and do not burn off as much energy as other children.

Learning difficulties and a delay in development

Most children with Prader-Willi syndrome have mild to moderate learning difficulties with a low IQ.

This means it will take longer for a child with Prader-Willi syndrome to reach important developmental milestones. For example, a child with the syndrome will typically begin sitting up at around 12 months and start walking at around 24 months. Many are late to talk and have difficulty pronouncing some sounds.

Most children with Prader-Willi syndrome can attend a mainstream school with extra support, although some will move to a special school at secondary level.

Their short-term memory is usually poor, although they often have a good long-term memory.

Despite these problems, children with Prader-Willi syndrome tend to be good at solving puzzles, such as word searches and jigsaws.

Read more about learning disability.

Short stature

Children with Prader-Willi syndrome are usually much shorter than other children of their own age. This is usually apparent by the time they're 2 years old.

Low levels of human growth hormone (HGH) contribute to the short stature, and they will not go through the typical growth spurt during puberty.

If HGH is not replaced, the average adult height for people with Prader-Willi syndrome is 159cm (5ft 2in) for a man and 149cm (4ft 10in) for a woman.

Children can be prescribed an artificial version of HGH to encourage growth (see managing Prader-Willi syndrome).

Behavioural problems

Most children with Prader-Willi syndrome have difficult behaviour traits such as tantrums and stubbornness. Typical behavioural problems include:

  • temper tantrums and sudden rages, which can be triggered by minor events
  • stubbornness and being very argumentative
  • controlling and manipulative behaviour
  • asking the same question over and over again, or frequently returning to the same topic in a conversation
  • repeating the same activity (such as playing the same game or watching the same video repeatedly)
  • keeping a very strict daily routine – any unexpected disruption to the routine can upset them

While these behavioural problems can be challenging for parents to deal with, children with Prader-Willi syndrome also have many positive aspects to their personality, such as being affectionate, kind, caring and funny.

Skin picking

Many children with Prader-Willi syndrome persistently pick their skin, usually their face, hands or arms. They may scratch, pierce or pull at their skin, sometimes using paper clips or tweezers. It can result in open sores, scarring and infection.

Some children may pick out pieces of poo from their bottom. As well as causing embarrassment for children and parents, this can result in bleeding and infection.

Sleep problems

Many children with Prader-Willi syndrome have problems sleeping. They can become tired and sleep during the day but then wake during the night or very early in the morning.

Some children have interrupted breathing during sleep (sleep apnoea). These episodes cause excessive sleepiness in the day, which only worsens inactivity and obesity.

Sleep apnoea gets worse as you gain weight, leading to a vicious cycle. It can also be dangerous to have interrupted breathing at night.

It's important to identify sleep apnoea because effective treatment and support is available.

High pain and vomiting tolerance

Children with Prader-Willi syndrome have a high tolerance for pain, which can be potentially dangerous.

For example, a serious condition such as appendicitis would cause severe pain in most people but may pass unnoticed or be a minor upset for a child with Prader-Willi syndrome.

Therefore, if a child with Prader-Willi syndrome does complain of pain, it should be taken seriously.

Abnormal curvature of the spine

Poor muscle tone means there's an increased chance of developing a curvature of the spine (scoliosis).

Other health problems

Children with Prader-Willi syndrome can develop:

  • eye problems – short or long-sightedness, or squint
  • poor muscle tone in the gut, which can cause constipation or a swollen tummy
  • tooth decay because they do not produce much saliva
  • weakening of the bones (osteoporosis) when they become adults because they do not have the sex hormones needed to keep bones healthy