You have not written the exact nature of your previous child's disability, myopathies / muscular dystrophies are of various types and obviously have different causes. The types of inheritance of each disorder is different and therefore you need to identify the exact nature of your first child's illness. Paediatricians who see children normally maybe unable to diagnose this. You may need a geneticist and a specialist paediatrician who deals with accessing the nature of these disorders and also any morphological changes seen, what we call a syndrome search. In other words a precise diagnosis of the child is required following which see a geneticist to know what kind of inheritance pattern the disease has. Then see if a prenatal (before birth) diagnosis is possible, and then do what you are told by a genetic counselor. Off hand I can only tell you that if this disease is inheritable then it generally occurs with an incidence like 25% to at the most 50%. Either way a lot of these disorders can be diagnosed before birth and the needful can be done. Going through a pregnancy without proper genetic counselling would not be the right thing to do.