How do healthcare providers diagnose Prader-Willi syndrome (PWS)?

In many cases of Prader-Willi syndrome, diagnosis is prompted by physical symptoms in the newborn.

If a newborn is unable to suck or feed for a few days and has a "floppy" body and weak muscle tone, a health care provider may conduct genetic testing for Prader-Willi syndrome.1,2,3 Formal diagnostic criteria for recognizing Prader-Willi syndrome depend on the age of the individual-specifically, whether the third birthday has been reached. Before age 3, the most important symptom is extremely poor muscle tone, called hypotonia, which makes infants feel floppy. In affected children 3 years of age and older, other symptoms become apparent, such as obesity, intellectual delays, learning disabilities, or behavior problems, especially connected with food and eating.3

  • Children younger than 3 years must have at least four major criteria and at least one minor criterion for a Prader-Willi syndrome diagnosis.
  • Those older than 3 years must have at least five major criteria and at least three minor criteria for a diagnosis of Prader-Willi syndrome.

Major Clinical Criteria of Prader-Willi Syndrome

  • Extremely weak muscles in the body's torso
  • Difficulty sucking, which improves after the first few months
  • Feeding difficulties and/or failure to grow, requiring feeding assistance, such as feeding tubes or special nipples to aid in sucking
  • Beginning of rapid weight gain, between ages 1 and 6, resulting in severe obesity
  • Excessive, uncontrollable overeating
  • Specific facial features, including narrow forehead and downturned mouth
  • Reduced development of the genital organs, including small genitalia (vaginal lips and clitoris in females and small scrotum and penis in males); incomplete and delayed puberty; infertility
  • Developmental delays, mild-to-moderate intellectual disability, multiple learning disabilities

Minor Clinical Criteria of Prader-Willi Syndrome

  • Decreased movement and noticeable fatigue during infancy
  • Behavioral problems-specifically, temper tantrums, obsessive-compulsive behavior, stubbornness, rigidity, stealing, and lying (especially related to food)
  • Sleep problems, including daytime sleepiness and sleep disruption
  • Short stature, compared with other members of the family, noticeable by age 15
  • Light color of skin, eyes, and hair
  • Small hands and feet in comparison to standards for height and age
  • Narrow hands
  • Nearsightedness and/or difficulty focusing both eyes at the same time
  • Thick saliva
  • Poor pronunciation
  • Picking of the skin

Additional Findings

  • High pain threshold
  • Inability to vomit
  • Curvature of the spine (scoliosis)
  • Earlier-than-usual activity in the adrenal glands, which can lead to early puberty
  • Especially brittle bones (called osteoporosis)

Genetic testing must confirm the Prader-Willi syndrome diagnosis. Almost all individuals with Prader-Willi syndrome have an abnormality within a specific area of chromosome 15.4 Early diagnosis is best because it enables affected individuals to begin early intervention/special needs programs and treatment specifically for Prader-Willi symptoms.

Genetic testing can confirm the chance that a sibling might be born with Prader-Willi syndrome. Prenatal diagnosis also is available for at-risk pregnancies-that is, pregnancies among women with a family history of Prader-Willi syndrome abnormalities.

Genetic Counseling and Testing of At-risk Relatives

Genetic counseling and testing provide individuals and families with information about the nature, inheritance, and implications of genetic disorders so that they can make informed medical and personal decisions about having children. Genetic counseling helps people understand their risks. The risk of occurrence in siblings of patients with Prader-Willi syndrome depends on what caused the disorder to occur.2

To locate a genetics or prenatal diagnosis clinic, see the GeneTests Clinic Directory.


  1. Gunay-Aygun, M., Schwartz, S., Heeger, S., O'Riordan, M. A., & Cassidy, S. B. (2001). The changing purpose of Prader-Willi syndrome clinical diagnostic criteria and proposed revised criteria. Pediatrics, 108, e92. Retrieved June 13, 2012, from external link
  2. Holm, V. A., Cassidy, S. B., Butler, M. G., Hanchett, J. M., Greenswag, L. R., Whitman, B.Y., et al. (1993). Prader-Willi syndrome: Consensus diagnostic criteria. Pediatrics, 91, 398-402.
  3. Cassidy, S. B., & Schwartz, S. (2009). Prader-Willi syndrome. In Pagon, R. A., Bird, T. D., Dolan, C. R., Stephens, K., Adam, M. P. (Eds.). Gene reviews. Seattle, WA: University of Washington. Retrieved June 13, 2012, from
  4. Butler, M.G., Kimonis, V., Dykens, E., Gold, J.A., Miller, J., Tamura, R., & Driscoll, D.J., (2018). Prader-Willi syndrome and early-onset morbid obesity NIH rare disease consortium: A review of natural history study. American Journal of Medical Genetics. Part A. 176(2), 368-375. Retrieved May 16, 2018, from
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