Down syndrome masked by Marfan syndrome in a neonate.

A male neonate with maternally inherited Marfan syndrome was also diagnosed with Down syndrome at 3 weeks of age. To our knowledge this is the first described case in the literature of the co-occurrence of Down syndrome and Marfan syndrome in a neonate. The diagnosis of Down syndrome was delayed and we hypothesise that Marfan syndrome had masked the usual phenotypic features of Down syndrome.