Babies can be born missing individual muscles or groups of muscles, or muscles can be incompletely developed.
Defects in muscles can occur alone or as part of a syndrome.
The pectoralis major muscle (the major muscle in the chest, sometimes called the "pecs" in adults) can be partly developed or missing altogether. This defect can occur alone or with various hand, breast, or nipple abnormalities, as in Poland syndrome.
One or more layers of the abdominal muscles may be missing at birth, as in prune-belly syndrome. In this disorder, the missing layers of abdominal muscles cause the abdominal wall to have a wrinkled appearance, the testes do not descend into the scrotum (cryptorchidism), and/or urinary tract defects develop. The cause of this disorder is not clear.
Treatment of muscle defects depends on how severe the muscle defect is. The infant may need reconstructive surgery.