The paranasal or accessory nasal sinuses begin their development as evaginations of the mucosa during the 3rd and 4th fetal months, but undergo their major expansion afterbirth according to the development of the facial cranium and the teeth. While in the newborn the sphenoidal and frontal sinuses are still only pre-dispositioned, the ethmoidal labyrinth and particularly the maxillary sinus are of appreciable size. In the 4-year-old the frontal and sphenoidal sinuses are already developed and in the 12-year-old the paranasal sinuses reach their final form and a size equivalent to the adult.