Spinal muscular atrophy
It can already be observed before birth. It is the most rare and most severe form of the condition. Infants who have it  tend to move less in the womb. As a result, they are often born with joint deformities or contractures. They have very weak muscle tone (hypotonia) at birth. Their respiratory muscles are also very weak. Most of them do not survive past infancy due to respiratory failure. Some also develop congenital heart defects.