Testing for Prader-Willi syndrome starts with a simple blood test that analyzes DNA methylation patterns on chromosome 15. This single test catches over 99% of all cases, making it the recommended first step whenever a doctor suspects the condition. From there, additional genetic tests can identify the specific cause, which matters for understanding recurrence risk in future pregnancies.
Signs That Prompt Testing
The clinical features that raise suspicion of Prader-Willi syndrome (PWS) vary dramatically by age, so doctors look for different red flags depending on when concerns arise.
In newborns, the hallmark is severe low muscle tone (hypotonia) with a poor or absent suck reflex. These babies are often unusually floppy, have a weak cry, show decreased movement, and are difficult to feed. Many need assisted feeding in the first weeks of life and gain weight slowly despite caregivers’ best efforts. When a newborn presents with this combination and no other obvious cause, PWS testing is typically one of the first genetic tests ordered.
In older children and adults, a clinical scoring system helps doctors decide whether testing is warranted. Major criteria, each worth one point, include infantile hypotonia, feeding problems in infancy, rapid weight gain between ages one and six, distinctive facial features, underdeveloped genitals, developmental delays, and an excessive, insatiable appetite. Minor criteria, worth half a point each, include decreased fetal movement, sleep disturbances, short stature, light skin and hair compared to family members, small hands and feet, and behavioral problems like skin picking or temper tantrums. For children under three, five points (with at least four from major criteria) justify genetic testing. For anyone older, eight points are needed, with at least five from major criteria.
DNA Methylation Analysis: The First Test
The gold standard first-line test is DNA methylation analysis, performed on a standard blood sample. It works because the chromosome 15 region involved in PWS carries a chemical “tag” (a methylation pattern) that differs depending on whether it came from the mother or the father. In PWS, the father’s copy of this region is either missing or silenced, so the methylation pattern looks abnormal in a specific, recognizable way.
This test has a clinical sensitivity greater than 99% for PWS. It detects all three major genetic causes: deletions on chromosome 15, cases where a child inherits both copies of chromosome 15 from their mother (called maternal uniparental disomy), and rare imprinting defects that silence the paternal genes. Results typically come back within 7 to 10 days.
A normal methylation result essentially rules out PWS. An abnormal result confirms it, but doesn’t tell you which of the three genetic mechanisms caused it. That distinction requires further testing.
Identifying the Specific Genetic Cause
Once methylation analysis confirms PWS, the next step is figuring out which type your child has. About 70% of cases result from a deletion on chromosome 15, roughly 25% from maternal uniparental disomy, and the remaining 4% from imprinting center defects. Each has different implications for family planning.
Deletion Testing
Chromosomal microarray is the preferred method for detecting deletions. It uses thousands of tiny DNA probes to scan for missing segments and can precisely measure the size and boundaries of the deleted region. Most PWS deletions fall into two types: a larger deletion of about 6 million base pairs (Type I) and a slightly smaller one of about 5.5 million base pairs (Type II). Microarray can also pick up atypical deletions, including some that are 50% larger than expected.
An older method called FISH (fluorescence in situ hybridization) uses a single fluorescent probe to check whether a specific segment of chromosome 15 is present or absent. It was the standard in the 1990s and still works for confirming typical deletions, but it can’t measure deletion size precisely, can’t detect atypical breakpoints, and misses uniparental disomy entirely. Microarray has largely replaced it.
Uniparental Disomy Testing
If no deletion is found, the lab tests for maternal uniparental disomy. This requires blood samples from both parents and the child (a “trio”). The lab examines short repeated DNA sequences called STR markers along chromosome 15 and traces which parent each copy came from. If all markers match the mother and none match the father, uniparental disomy is confirmed. Results for this test take about two weeks. Chromosomal microarray with SNP probes can also suggest uniparental disomy by showing long stretches where both copies of chromosome 15 are identical.
Imprinting Center Defects
If methylation is abnormal but there’s no deletion and both parents contributed a copy of chromosome 15, an imprinting center defect is assumed. These account for only about 4% of PWS cases and involve either a tiny deletion within the imprinting control region (too small for standard microarray to detect) or a random failure to set the correct methylation pattern (called an epimutation). Specialized techniques like droplet digital PCR or whole-exome sequencing can sometimes identify the specific type of defect, though these tests are available only at select labs.
Why the Genetic Subtype Matters
Knowing the exact genetic mechanism is critical for one reason above all: recurrence risk. Most deletions and uniparental disomy cases occur randomly, meaning the chance of having another child with PWS is very low, generally under 1%. Imprinting center defects are different. If the father carries a heritable imprinting center deletion, each future pregnancy has a 50% chance of resulting in PWS. This makes subtype identification essential for any family considering additional children.
Prenatal Testing
PWS can be detected before birth, though it’s rarely the reason prenatal testing is initiated. More often, it’s discovered incidentally during testing prompted by other concerns.
Chorionic villus sampling (CVS), done around 10 weeks of pregnancy, and amniocentesis, done later, can both provide fetal DNA for analysis. If standard chromosome testing during either procedure reveals trisomy 15 mosaicism (three copies of chromosome 15 in some cells), this is a specific red flag. The fetus may have started with three copies and “corrected” to two, but if both surviving copies came from the mother, the result is uniparental disomy and PWS. Any time trisomy 15 mosaicism shows up on CVS or amniocentesis, further molecular testing for PWS should follow.
DNA methylation analysis can be performed on amniotic fluid cells, just as it is on blood after birth. In one study of 43 amniocentesis samples suspected of having a 15q11-q13 deletion, detailed molecular testing confirmed the deletion in 7% of cases. Prenatal testing is most commonly pursued in families with a known imprinting center defect or a chromosomal rearrangement involving chromosome 15, where the recurrence risk is significantly elevated.
What To Expect From the Process
Testing is ordered by a geneticist, pediatric neurologist, or sometimes a neonatologist in the case of a floppy newborn. The initial methylation test requires only a blood draw, and results come back in about one to two weeks. If the result is positive, follow-up testing to determine the subtype adds another two to four weeks depending on the lab and which tests are needed.
No special preparation is required for the child. For uniparental disomy testing, both parents will also need to give a blood sample so the lab can compare the family’s DNA. Genetic counseling is a standard part of the process, both to explain results and to discuss what the specific subtype means for the child’s care and for future family planning.