Babies are born with a natural reflex to suck, natures way of ensuring food intake. Sucking is also soothing and tends to induce sleep, as evidenced by the way young infants drift off to sleep as they suck at bottle or breast. Fussy babies often learn to soothe themselves by sucking on their thumb. Some parents encourage this in young babies, because the thumb is always available, unlike a pacifier which can be lost, causing distress for the infant who cant get the pacifier back in its mouth. Other parents prefer to use a pacifier, partly because they think it will be easier to break the pacifier habit than thumb-sucking, partly because even newborns can be given a pacifier, but arent yet coordinated enough to get their thumb in their mouth on demand.
Vigorous sucking can push teeth out of their natural alignment, although this is more likely to occur with a thumb. Most pacifiers are designed by orthodontists to try and avoid the kind of damage that has been caused in the past. Babies who suck more gently don't typically have as many problems as those who suck harder. The position of the thumb also has an effect on the damage that can occur. Changes to the roof of mouth have been noted in some children due to the intensity and frequency of thumb-sucking.
Most toddlers stop on their own between two and four, and up until this age, it is not recommended to make an issue of it. By the age of four, however, parents should begin to work with the child to break the habit. Children who are still sucking their thumbs when their permanent teeth come in can cause lasting damage to their teeth.
It is easier to break a pacifier habit than thumb-sucking, because the thumb is always handy, while the parent is able to remove the pacifier. The easiest way to break a pacifier habit is in small steps. Keep it out of sight; remind the child that they are big kids. Use it only when sleeping, then remove at nap time, and use only at night, then never. In order to break the thumb-sucking habit, the parent needs to monitor the child, using encouragement and praise. Help your child to overcome insecurity by making sure they know you are on their side, and you are doing this together, not being demanding or impatient.