A retained tooth is a deciduous or baby tooth that is still present in the mouth after its replacement permanent or adult tooth has erupted. This usually occurs because something went wrong with the process of root resorption, so that the tooth root is either incompletely absorbed or does not resorb at all. When this happens, the baby tooth occupies the place in the mouth that is meant for the permanent tooth, forcing the permanent tooth to erupt at an abnormal angle or in an abnormal position. The end result is crowding or malocclusion of the teeth, causing an abnormal bite, accidental bites into palates or an abnormal jaw position.
Most obvious sign is the presence of primary or deciduous teeth after eruption of permanent teeth. Other signs include
Bad breath (halitosis)
Abnormally-positioned permanent teeth
Swollen, red, bleeding gums around baby teeth
Local gingivitis and periodontal disease due to teeth overcrowding
A permanent abnormal passageway between the mouth and nasal cavity (oronasal fistula)