Abstract: A sleep sack system for treating or preventing a condition, characterized by abnormal head shape, in an infant. The sleep sack system includes an anterior infant pouch for containing a portion of an infant's body, a wedge, and a posterior wedge pouch, located on a back portion of the anterior infant pouch, for containing the wedge. When worn by an infant, the sleep sack system causes a specific portion of the infant's head to contact a sleeping surface, thereby generating a mild but consistent pressure to alter a shape of the infant's head. The invention is useful for correcting an abnormal head shape and treating or preventing a condition characterized by an abnormal head shape. The invention is particularly useful for treating or preventing plagiocephaly.