Infant thumb sucking is a natural reflex, performed repetitively, automatically, and without nutritional value. It is, however, a palliative act, making the infant feel happy and secure. Thumb sucking is relaxing and may help induce sleep, explaining why children suck thumbs as they fall asleep. Of Interest, sonograms of pregnant women have noted fetal thumb sucking in the womb. It is postulated that the fetal thumb sucking reflex establishes neurological pathways crucial for future breast feeding and for speech development. Correct facial muscle training of the jaws and tongue shape commence in the womb with the proper thumb suck by the fetus. Apparently, prior to birth of the infant the game plan for normal upper and lower dental arches with their teeth and for speech development is already set into play.
Persistent thumb sucking, however, may adversely affect facial muscle movements, as muscles in the lips and buccal area are preferentially used while other muscles (e.g., the masseter muscle involved in food chewing) are not as active. Also affected is the tongue, which may protrude through the front teeth (incisors) during swallowing, speech, and when the tongue is at rest. Initially, all infants exhibit a swallowing pattern involving thrusting the top of the tongue forward against the lips (termed, “tongue thrust”). But by six months of age this swallowing reflex is replaced by a mature swallowing to allow for the ingestion of solid foods. Prolonged thumb sucking may lead to persistent tongue thrust which, in turn, is accompanied by problems with teeth alignment; the end result is a problem in speech articulation. Specifically, sounds that are distorted by the inaccurate placement of the tongue include the tongue tip sounds, /s/,/ z/,/ t/,/ d/, /l/,/sh/,/zh/, /ch/, /j/ and /n/. For example, if the child pronounces an /s/ like a /th/, instead of pronouncing “some,” the child may say “thumb.”
Past the age of six years, roughly the time when permanent teeth erupt, thumb sucking may lead to malocclusion, or, a slight misalignment of the teeth. Vigorous thumb sucking puts much pressure on the soft tissue of the roof of the mouth and on the sides of the jaw, causing the jaw to narrow and the top and bottom teeth not to properly meet. In addition, the thumb sucking may cause the top teeth to push out over the lower teeth (resulting in “buck teeth”).
Thumb sucking habits are prevalent among children and, according to the American Dental Association (ADA), in most cases thumb sucking spontaneously ceases between the ages of two and four years of age. Peer pressure causes many school-aged children to stop placing their thumbs in their mouths.
Behavioral interventions for thumb sucking include aversive conditioning by applying a bad-tasting substance on the thumb, by wrapping the thumb with an adhesive bandage, or by putting cotton gloves on your child’s thumb. Conversely, positive reinforcement, such as offering praise, when the child avoids the habit may help the child break the habit. Punishing or shaming a child for thumb sucking only leads to less self esteem and is psychologically harmful.
Habitual thumb sucking that persists beyond four years of age, despite behavioral interventions, may require the involvement of a pediatric dentist or interceptive orthodontics to prevent malocclusions.