Duchenne muscular dystrophy (DMD) is a progressive form of muscular dystrophy that occurs primarily in males, though in rare cases may affect females. DMD causes progressive weakness and loss (atrophy) of skeletal and heart muscles. Early signs of DMD may include delayed ability to sit, stand, or walk and difficulties learning to speak. Muscle weakness is usually noticeable by 3 or 4 years of age and begins in the hips, pelvic area, upper legs, and shoulders. The calves may be enlarged. Children with DMD may have an unusual walk and difficulty running, climbing stairs, and getting up from the floor. DMD may also affect learning and memory, as well as communication and certain social emotional skills. Muscle weakness worsens with age and progresses to the arms, legs and trunk. Most children with DMD use a wheelchair full time by age 13. Heart and respiratory muscle problems begin in the teen years and lead to serious, life threatening complications.