Thumb sucking is a natural reflex that begins in the womb. Babies in utero begin sucking their thumb in the second trimester – even as early as 15 weeks of gestation. Sucking is a reflex that babies use to soothe and calm themselves. It makes them feel happy and secure. Babies use sucking to learn about the world around them. Infants suck on pacifiers, thumbs, fingers, and other objects.
Toddlers and young children may suck their thumbs to soothe and comfort themselves. Because thumb sucking is relaxing, it helps babies and young children fall asleep. That’s why you may notice that your toddler or young child starts thumb sucking when they’re getting tired. Thumb sucking allows children to fall asleep more easily, and it helps them sleep through the night at an earlier age than children who don’t thumb suck.