Editor—Myotonic dystrophy (DM) is a common autosomal dominant disorder characterised by myotonia, muscle weakness, ECG abnormalities, cataracts, hypogonadism, and frontal balding in the typical adult form (MIM 160900). The genetic defect consists of the amplification of an unstable CTG trinucleotide repeat in the 3′ untranslated region of the dystrophia myotonica protein kinase gene (DMPK), which maps to 19q13.3.1 2 Normal subjects have five to 37 repeat copies while affected subjects have over 50 repeats.1 There is some correlation between repeat length and clinical symptoms, especially with respect to the age at onset.3-6 In the vast majority of cases, the number of repeats increases during parent-offspring transmission of the mutant allele, thus providing some molecular basis to the observation of anticipation (increased severity of the disease in successive generations).7 8 However, a decrease in repeat size is occasionally observed in the offspring, mostly in the case of paternal transmission of an expansion of over 600 trinucleotide repeats,9 but contraction of a parental expanded repeat back to the normal range when transmitted to offspring seems to be an extremely rare phenomenon.10-12 Here we report such a case and emphasise the direct impact of this situation on prenatal diagnosis (PND) of DM.