Objectives: We have previously shown that Duchenne muscular dystrophy (DMD) can be diagnosed by fetal muscle biopsy and immunohistochemical staining showing the absence of dystrophin. In Becker muscular dystrophy (BMD), there is a variable dystrophin staining pattern. Study design: A 26-year-old, gravida 5, para 1, SAB 2, VIP 1, white female had a son with the diagnosis of BMD, with no other affected family members. In the current pregnancy, the male fetus inherited the same X chromosome. Results: Fetal muscle biopsy revealed no dystrophin, consistent with the diagnosis of DMD, prompting reanalysis of the sibling whose diagnosis was then changed to DMD. Conclusions: Molecular techniques in prenatal diagnosis in this case not only led to the diagnosis of a severe disorder in the current pregnancy, but changed the diagnosis in another child, allowing for more appropriate counseling of this couple.