Why the child suck thumb
Babies generally suck their thumb to soothe themselves, which is good, because they need to learn how to rely on their own resources. The ability to regulate or control one's behavior and emotions is an important developmental challenge.
Toddlers suck their thumbs because it's comforting and calming. Your toddler probably practiced this habit while he was still in the womb and perfected it as an infant. Now they are may turn to his thumb when they are tired, scared, bored, sick, or trying to adjust to challenges such as starting preschool or going on a long car ride. They are may also use his thumb to fall asleep at bedtime and to lull himself back to sleep when they are wakes up in the middle of the night. Children who rest their thumb passively in their mouth are less likely to have difficulty than children who suck aggressively. Watch your child and analyze his technique. If he sucks vigorously, you may want to begin curbing his habit earlier, say at age 4.
You don't have to stop your child from sucking her thumb right now. In general, thumb-sucking is more irritating to parents than it is harmful to kids. Your baby will stop when she's ready and has developed other ways of soothing herself.