Your baby has been sucking his/her thumb since he/she was in your womb. This trait continues for a while, thereafter. Sucking the thumb is a mechanism to show various emotions that they are unable to put forth. When your child is scared, nervous, hungry or sleepy, their immediate calming mechanism is this. At an early stage, every baby has the tendency to suck their thumb. Do not stop your child from doing the same, as this will put them off guard.
Thumb sucking is the best means to teach them about self-soothing. The moment you force them to stop, they will be intrigued by it and do it even more. Compelling them to do anything will turn them into rebels from an early age.
Instead, keep track of the moments when your baby puts his/her thumb into their mouth. Once you know their routine ensure that you kerb the habit gradually. When they put their thumb in the mouth, try and distract them. If it in front of the TV, give them a squishy ball to play with or engage their fingers in something to keep them off sucking their thumb. If your baby sucks their thumb before sleeping tries and distracts them with a song or story to help them sleep.
Instead of worrying about your child sucking their thumb, observe the intensity with which your baby is sucking his/her thumb. At an early age, they tend to do it lightly, but over the years, they might start to intensify their tongue and teeth movements. Keep track of your child's finger movements. If you notice the intensity is quite stiff, try and get your baby to drop the habit before the age of 4.
Once the permanent teeth start to grow, your child may damage the shape of their tooth and jaw due to this. Encourage them to overcome this habit before they hit the age of 6 years. Even though your child might seem determined to continue the habit, try and suppress it with an encouragement of all sorts.
Keeping your child's fingers distracted at times you know they’ll suck their thumb. This should be done before their permanent teeth set in. Remember, never to use any harsh methods to get your child to drop the habit, as it will make things worse.