British Columbia

Prader-Willi Syndrome Association

BCPWSA is here for you.

We are a non-profit charity that provides an understanding and awareness of Prader-Willi Syndrome by supporting those who have the syndrome, their families, and all those who come in contact with PWS.

Learn more about how you can receive help and support.

Become a member.

If you or a family member is affected by PWS, we encourage you to become a member of our association.

What is Prader-Willi Syndrome?

What is PWS?

Prader-Willi Syndrome (PWS) is a complex genetic disorder that typically causes low muscle tone, short stature, incomplete sexual development, cognitive disability, challenging behaviours and a chronic feeling of hunger that can lead to excessive eating and life-threatening obesity.

Click this button to learn more about Prader-Willi Syndrome.

How common is PWS?

It is estimated that 1 in 15,000 people are born with PWS. PWS affects males and females with equal frequencies and affects all races and ethnicities. The occurrence of PWS is caused by a lack of genetic material in a particular region of chromosome 15 (15q11-q13). Individuals typically inherit one copy of chromosome 15 from their mother and one copy of chromosome 15 from their father. Normally, the genes in the PWS region are only active in the chromosome copy from the father. PWS can occur in three different ways, due to this genetic defect causing the inactivity of the paternal chromosome 15:

PWS dy deletion

In the majority of cases (70%), there is a small deletion – the critical genes are somehow lost or deleted from the chromosome normally inherited from the father.


In most of the remaining cases (just under 30%), the entire chromosome from the father is missing and there are instead two chromosome 15s inherited from the mother. This is termed uniparental disomy. (UPD15)

PWS by Imprinting Defect

In a very small amount of cases (1-3%), a small genetic mutation in the PWS region causes the paternal chromosome 15 genetic material to be inactive (even though it is actually there).


Prader-willi syndrome is diagnosed with a blood test that looks for the genetic abnormalities that are specific to PWS. This is called a methylation analysis. It can detect all types of PWS and is the preferred test for diagnosis. A FISH test (fluorescence in-situ hybridization) can identify PWS by deletion, but it does not diagnose other types of PWS. If a methylation test is done first, additional testing may be needed to determine whether PWS is caused by a paternal deletion, UPD, or an imprinting mutation. In the case where an imprinting mutation is suspected, blood testing may also be done from the parents.

Does the overeating associated with PWS start at birth?

No. In fact, newborns with PWS often cannot get enough nourishment because low muscle tone impairs their ability to suckle. Many require special feeding techniques or tube feeding for several months after birth, until muscle control improves. Sometime, in the following years, usually before school age, children with PWS develop an intense interest in food and can quickly gain excess weight if calories are not monitored.

Research is always happening, but there are several stages to consider when caring for a person with PWS. In the past, PWS was only described as a 2 stage syndrome – failure to thrive (poor feeding and slow growth) followed by hyperphagia (increased appetite). After observing that the changes in appetite and weight gain which occur in PWS are more gradual and complex than traditionally described, a research team investigated and presented an updated clinical description of the nutritional phases of PWS. Their research was published in 2011 outlining 5 major phases, including some sub-phases.

Phases of PWS

Phase Ages Characteristics

0 prenatal-birth Decreased fetal movements and lower birth weight than siblings

1a 0-9 months Hypotonia with difficulty feeding and decreased appetite

1b 9-25 months Improved feeding and appetite; growing appropriately

2a 2.1-4.5 years Weight increasing without appetite increase or excess calories

2b 4.5–8 years Increased appetite and interest in food, but can feel full

3 8 years–adult Hyperphagic; rarely feels full

4 adulthood Appetite is no longer insatiable (only very few adults)

*A more detailed version can be found below.

PWS is a spectrum disorder and symptoms vary in severity and occurrence among individuals.

Phases of Prader-Willi Syndrome - detailed

Phase 0

Decreased fetal movements and lower birth weight

Full-term birth weight and BMI are about 15–20% less than the siblings

Typically normal gestational age

85% have decreased fetal movements

Phase 1a

Hypotonia with difficulty feeding (0–9 months)

Weak, uncoordinated suck. Usually cannot breastfeed

Needs assistance with feeding either through feeding tubes (nasal/oral gastric tube or gastrostomy tube) or orally with special, widened nipples. Many would die without assisted feeding

Oral feeds are very slow

Severely decreased appetite. Shows little or no evidence of being hungry

Does not cry for food or get excited at feeding time

If feeding just occurred when baby “acted hungry” then would have severe “failure-to-thrive”

Weak cry

Phase 1b

No difficulty feeding and growing appropriately on growth curve (9–25 months)

No longer needs assisted feeding

Growing steadily along growth curve with normal feeding

Normal appetite

Phase 2a

Weight increasing without an increase in appetite or excessive calories (2.1–4.5 years)

Infant starts crossing growth curve centile lines

No increase in appetite

Appetite appropriate for age

Will become obese if given the recommended daily allowance (RDA) for calories or if eating a “typical” toddler diet of 70% carbohydrates

Typically needs to be restricted to 60–80% of RDA to prevent obesity

Phase 2b

Weight increasing with an increase in appetite (4.5–8 years)

Increased interest in food. Frequently asking “food related” questions

Preoccupied with food. Very concerned about the next meal/snack (e.g., “Did you remember to pack my lunch?”)

Increased appetite

Will eat more food than a typical child if allowed

Will eat food within their line of sight if unattended

Will become obese if allowed to eat what they want

Can be fairly easily redirected about food

Can feel full

Will stop eating voluntarily

Phase 3

Hyperphagic, rarely feels full (8 years adulthood)

Constantly thinking about food

While eating one meal they are already thinking about the next meal

Will awaken from sleep early thinking about food

Will continue eating if portion size is not limited

Rarely (truly) feels full

Will steal food or money to pay for food

Can eat food from garbage and other unsavory/inedible sources (e.g., dog food, frozen food, crayons, etc.)

Typically are not truthful about what they have eaten (i.e. amount and types of food)

Will gain considerable amount of weight over a short period of time if not supervised (e.g., some individuals are known to have gained up to 20 pounds in one weekend)

Food typically needs to be locked up. Frequently the child will ask the parent to lock the food if the parent has forgotten

Will break into neighbors’ houses for food

Temper tantrums and “meltdowns” frequently related to food

Needs to be placed on a diet that is approximately 50–70% of the RDA to maintain a healthy weight

Phase 4

Appetite is no longer insatiable (adulthood)

Appetite may still be increased or may be normal or less than normal

Previously in phase 3, but now a noticeable improvement in their appetite control

Can feel full

Appetite can fluctuate in this phase, but the key component is noticeable improvement in control of appetite compared to when they were younger

Not as preoccupied with food

Absence of major temper tantrums and “meltdowns” related to food

Onset in adulthood. Could be as early as 20s or as late as 40–50s

Most adults have not gone into this phase and maybe some (most?) never will

What causes the appetite & obesity STRUGGLES in PWS?

People with PWS have a flaw in the hypothalamus part of their brain, which normally registers feelings of hunger and satiety. While the problem is not yet fully understood, it is apparent that people with this flaw never truly feel full; they have a continuous urge to eat that they cannot learn to control. To compound this problem, people with PWS need less food than their peers without the syndrome because their bodies have less muscle and tend to burn fewer calories. Unfortunately, no appetite suppressant has worked consistently for people with PWS. Most require a low caloric diet all their lives and must have their environments designed so that they have limited access to food.

What kinds of behavior STRUGGLES do people with PWS have?

In addition to their involuntary focus on food, people with PWS tend to have obsessive/compulsive behaviours that are not related to food, such a repetitive thoughts and verbalizations, collecting and hoarding possessions, picking at skin irritations, and a strong need for routine and predictability. Frustration or changes in plans can easily set off a loss of emotional control in someone with PWS, ranging from tears to temper tantrums to physical aggression.

What does the future hold for someone with PWS?

With supports, people with PWS can expect to accomplish many of the things their "non-PWS" peers do: complete school, achieve in their outside areas of interest, be a productive worker under the right conditions, even move away from their family home. They do, however, need a significant amount of support from their families, and from school, work and residential service providers to both achieve these goals and avoid obesity and the serious health consequences that accompany it. Even those individuals with IQs in the normal range need lifelong diet restrictions and protection from food availability.