The sucking reflex, like rooting, is an automatic survival action. Rooting actually helps the baby become ready to suck. When the roof of the baby's mouth is touched, the baby will begin to suck. This reflex begins at about the 32nd week of pregnancy and is not fully developed until about 36 weeks’ gestational age. Hence, premature babies may have a weak or immature ability to suck.
Babies also have a hand-to-mouth reflex that goes with rooting and sucking and may suck on fingers or hands. Sometimes the fetus can be seen sucking his or her thumb on a prenatal ultrasound.