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Background Chromosome abnormalities are a frequent finding in prenatal invasive testing for fetal malformations and/or growth retardation. Case presentation We present a case of low level (8%) mosaic trisomy 15 detected on amniocentesis after fetal heart anomalies and IUGR (intrauterine growth retardation) were found on routine scan. Postnatal karyotype confirmed a very low level (2%) mosaicism in the skin but not in blood lymphocytes or in the urine. Methylation specific testing of chromosome 15 showed maternal uniparental disomy and consequently the newborn was diagnosed with Prader-Willi syndrome (PWS). Conclusions This case illustrates the need of further genetic testing in all trisomy 15 mosaicisms detected in prenatal invasive testing in order to screen for PWS, a more frequent entity than trisomy 15, altogether providing appropriate genetic counseling and adequate clinical management. The recommendation is applicable to prenatally detected mosaic trisomies of other chromosomes carrying imprinted genes, such as 7, 11 and 14.