Prader-Willi syndrome (PWS) is the most common known genetic cause of life-threatening obesity in children.  Although the cause is complex, it results from an abnormality on the 15th chromosome.  It occurrs in males and females equally and in all races.  Prevalence estimates have ranged from 1:8,000 to 1:25,000 with the most likely figure being 1:15,000.

PWS has been historically known as a two stage syndrome, with the first stage characterized by hypotonia (low muscle tone) and poor feeding in infancy, and the second stage exhibiting hyperphagia (an uncontrollable drive to eat), combined with weight gain on fewer calories.  Current research through an extensive natural history study has shown that PWS is not just a two stage but a multiphase syndrome:

Occurs in utero and is characterized by decreased fetal movements, birth weight and length.

The infant is hypotonic and not obese.

  • Sub-phase 1a (median age range = 0-0.75 years) is characterized by poor appetite, feeding difficulties, and failure to thrive.
  • Sub-phase 1b (median age range = 0.75-2.08 years) occurs when the infant is growing steadily along the normal growth curve which is associated with improved appetite.

Occurs when the weight starts to increase and crosses percentile lines in the normal weight curve. This generally begins between 18-36 months of age.

  • Sub-phase 2a (median age range = 2.08-4.50 years) is when the child’s weight increases above the normal rate without significant increase in calories intake. During this phase, the children do not have an increased appetite or increased interest in food.
  • Sub-phase 2b (median age range = 4.5-8.0 years) occurs when the child has an abnormally increased, but not necessarily insatiable, appetite which leads to overweight/obesity if the diet is not appropriately regulated.

(Median age of onset is 8.0 years and lasts into adulthood) is the development of an insatiable appetite/drive to eat, accompanied by intense or relentless food-seeking. This is the classic phase most people typically associate with PWS, but its onset is actually quite variable in PWS. It may appear as early as 3 years of age or as late as 15 years of age. This phase can be managed by the presence of appropriate environmental interventions (usually including locked food access) and psychological food security.

May occur in later adulthood when an individual who was previously in phase 3 no longer has an insatiable appetite and can feel full. Families and care takers note a significant improvement in appetite and weight control. Phase 4 only occurs in a minority and most of adult individuals with PWS will remain in phase 3.

Other typical characteristics of Prader-Willi syndrome include low muscle tone, motor development delays, short stature (if not treated with growth hormone), and incomplete sexual development.  The average IQ is 70, but even those with normal IQ’s almost always have learning difficulties or differences. Children with PWS can be sweet and loving, but they display characteristics of the Prader-Willi personality that can cause social and behavioral problems.  They also have many complex and unique medical issues.

Basic Facts

Conditions Resembling PWS

Genetics of PWS

PWS Statistics

Getting Tested

Diagnostic Criteria

New Diagnosis

Personal Stories


Glossary of Terms