Children are born with a natural sucking reflex. In fact, babies begin to suck on their fingers while still in the womb. Sucking a thumb or pacifier is relatively harmless during the first few months of life and may actually be emotionally comforting to an infant. However, prolonged sucking that lasts into the preschool years or later may cause long-term oral complications including open bite, tongue thrusting, crooked teeth and improper jaw growth..