Neonates demonstrate what is termed a true suck. The tongue, jaw, cheeks, and lips work together as a unit. Up/down movement of the jaw is limited by the high rib cage, physiologic flexion, and fat pads in the cheek. The lips are positioned around the nipple without requiring active lip closure. The tongue cups around the nipple shaped by the fat pads and jaw. This is referred to as true suck or total sucking pattern, differentiating it from active suck which develops later when active and controlled lip, cheek, tongue, and jaw movement develop.
The development of antigravity extension, which begins with the head and neck, modifies the oral structures and their activity. The jaw moves up/down and forward/backward in large rhythmical movements to draw liquid from the bottle or breast. The tongue moves forward/back stroking or licking the nipple to draw liquid to the mouth. The lips surround the nipple but do not keep constant contact with the bottle or breast because of the movement of the jaw and tongue. Liquid is lost at the corners of the mouth.
Center of upper and lower lips actively hold nipple, tongue actively cups around nipple, excursion of up/down jaw movements is smaller and more controlled. At 3-5 months, infants make the transition from suckling to active sucking. Mature, active sucking complete and predominates by 6 months.