Prenatal diagnosis of Duchenne and Becker muscular dystrophy (DMD) is performed as a routine procedure in many laboratories. The major potential problem is an incorrect diagnosis that could be obtained due to contamination with maternal tissue. We report a case of mosaicism of the X-chromosomes confined to the placenta as a possible source of confusing results in prenatal diagnosis of DMD. To the best of our knowledge, this is the first reported case of this problem in a prenatal DMD diagnosis.