1. Newborns Open Their Mouths Before the Hand Arrives
Have you ever seen someone open their mouth just before taking a bite of food? A study published in 1995 by Lew and Butterworth found something similar in newborns [2]. When hungry, babies opened their mouths more often before their hands touched their faces than when their hands touched other parts of their face; this difference was not seen after eating [2].
Another study showed 2-day-old newborns videos of a thumb going into an open mouth and other videos without that action. The babies looked longer at the video where the thumb entered the mouth [3]. This paper also notes that during pregnancy, between weeks 24 and 36, fetuses touch their mouths with their hands more often [3].
These observations show that movements where the mouth opens before the hand reaches it have been observed. However, we cannot say for sure that babies do this on purpose because they want to eat. So, what are fingers doing once they reach the mouth?
2. Why Babies Put Everything in Their Mouths Around 5 Months
The Japan Pediatric Dental Society states that babies aged 2 to 4 months unconsciously suck on fingers or objects near their mouths [1]. By about 5 months, they start bringing everything to their mouth to suck on it [1].
This behavior is thought to be practice for coordinating eyes and hands. It also helps them learn about shape, taste, and texture [1]. Since the mouth is used for eating, it becomes a tool for exploring the world when hand skills are still developing.
Fingers are always within reach, so babies often suck on them. The sources do not explain exactly why this specific action happens. But why does finger sucking also appear when babies need to calm down?
3. Why Babies Suck Their Fingers When Sleepy or Bored
By age 1 or 2, children play with blocks, toy cars, and dolls. As these activities increase, daytime finger sucking decreases [1]. The Japan Pediatric Dental Society notes that it may only happen when a child is bored or sleepy [1].
Pediatricians view finger sucking as a natural part of growth. They believe it can help reduce anxiety and tension, especially in early childhood [1]. Because of this, many pediatricians hold the opinion that children should not be forced to stop the habit.
However, the sources do not explain exactly why sucking fingers creates a feeling of calm. Views on this differ among specialists. Where does this difference in opinion come from?
4. When Finger Sucking Decreases as Babies Grow
When babies learn to stand holding on, walk while holding on, and stand alone, finger sucking tends to decrease because it interferes with these movements [1].
Between age 3 and school entry, children get used to being away from parents and playing with friends. Finger sucking naturally fades during this time [1]. By age 5, most children do it very little. Rarely, some 6-year-olds still do it frequently day and night; in these cases, it may not disappear without special support [1].
A survey in one district of Tokyo (K Ward) from 2002 provides a rough idea of how common this is. It found that 28.5% of children at 14 months, 28.9% at 18 months, 21.6% at 2 years, and 20.9% at 3 years sucked their fingers [1]. This data is from one area about 20 years ago, not a national average.
Even with more hand-based play, some children continue the habit. How does this affect their teeth?
5. How Finger Sucking Affects Teeth, and Advice for Parents
The Japan Pediatric Dental Society states that long-term finger sucking can affect tooth alignment and bite [1]. Possible issues include upper front teeth sticking out, a gap between upper and lower front teeth, or crossbite. Pushing the tongue into gaps can also cause speech problems with certain sounds [1]. The effect depends on which finger is used and how it is sucked.
The society offers guidelines for parents. Until about 12 months, finger sucking is a normal physiological part of development, so observation is fine. From age 1 to 2, parents should not be overly nervous but watch the child’s overall life [1]. It is important to tell parents that stopping is not necessary until around age 3.
If frequent finger sucking continues after age 3, support from pediatricians, pediatric dentists, or clinical psychologists may be needed [1]. These guidelines are not meant to blame parents. They are tools to help decide when to consult a professional while watching the child’s condition.
6. How You Can Observe Babies or Read More About Finger Sucking
If you have a chance to meet a baby, ask permission from the caregivers first. Watch from a distance as the hand moves toward the mouth. Observe whether the mouth opens before the hand arrives [2]. Do not touch the baby; just observe. Remember that babies’ behavior changes depending on their mood, so do not judge by one look alone.
To read more, the Japan Pediatric Dental Society’s "Thoughts on Finger Sucking" is available as a Japanese PDF [1]. You can check the original text to see where different specialists have different views.