Before a baby has any teeth, the tongue sits between the two jaws so that the baby can swallow without liquid escaping from the mouth. Once the baby teeth come through, they provide the necessary barrier, and the tongue should instinctively move back behind the teeth. In some children, and sometimes even in adults, the habit of placing the tongue between the teeth or pushing them forward when swallowing persists as a faulty swallowing pattern. The tongue moves forward and the lips tighten to help push the food or liquid towards the throat. This tightening of the lip muscles is accompanied by wrinkling of the skin on the chin and is called a ‘lip seal’. If the child has an orthodontic check-up, this problem is picked up in time, before it has a lasting effect on the teeth and jaws. This type of swallowing is called ‘infantile’ because it is typical of early childhood. The problems this faulty pattern leads to are the teeth being pushed forward and an open bite. Sometimes the problem is combined with an incorrect tongue position at rest. In that case we can also expect a wide lower jaw, a narrow upper jaw and a crossbite.
This answer is for general information. Your own case needs an examination by an orthodontist.