Many children suck their thumbs. Some even start in the womb. Now a recent study suggests that children who suck their thumbs or a pacifier after age 2 may be affecting their teeth and jaws.

Researchers at the University of Iowa College of Dentistry followed 372 children from birth to age 5. They collected information on thumb or finger sucking and pacifier use, and took measurements in the children's mouths at regular time intervals.

Of the children, only 2.2 percent did not have some type of sucking habit. Among those that did, 32 percent had stopped the habit by age 1, 25 percent by age 2, 19 percent by age 3 and 9 percent by age 4. Sixteen percent still had the habit beyond age 4.

Children who sucked their thumb or used a pacifier until at least age 4 were more likely to have several differences in their mouths, compared with children who stopped these oral habits earlier in life:

• A narrower palate (roof of the mouth)
• Top front teeth that protrude
• Open bite, in which the top and bottom front teeth do not meet when the mouth is closed.
• Crossbite, in which the bottom teeth overlap the top teeth when the mouth is closed. Normally, the top teeth should slightly overlap the bottom ones.

Children who still had oral habits until age 2 or 3 also had changes in their mouths, compared with children who had quit sucking their thumbs or a pacifier by age 1. For example, crossbite occurred in about 6 percent of children who stopped by age 1 but in 13 percent of those who stopped by age 3 and in more than 20 percent of children who had an oral habit beyond age 4.

"A sucking habit is perfectly fine for an infant, but after that period it's not desirable," said John Warren, DDS, assistant professor at the University of Iowa and first author of the study. "We need to think about doing something about these habits earlier than we used to. The ideal time would be shortly after their first birthday, or at least prior to their second birthday."

Although the changes occurred in the children's baby teeth, "There's reason to believe that some of these changes will carry over into the permanent teeth," Warren said. "Some of the problems, like the open bite and the protrusive upper jaw, could require surgical treatment if they continue."

The researchers plan to revisit the children at age 8 or 9 to see if the changes persisted. By then, children usually have about 12 permanent teeth. However, some of the children may be receiving orthodontic treatment, which would skew the measurements.

Besides changes in the teeth, Warren said, the results suggest that a long-term sucking habit could narrow and elongate the palate. "We didn't take X-rays, but the results suggested there were some bony changes that took place," he said.

In another study, Warren and others compared sucking a pacifier to sucking a thumb or finger. They both cause problems if the habit is prolonged, he said, but the pacifier tends to produce crossbites, while thumb sucking causes more open bite and protrusive upper front teeth.

Thumb or finger sucking is harder to stop than pacifier use, Warren said. "Keep your child from developing a thumb or finger-sucking habit if you can. The pacifier habit is preferable, but the best scenario is neither one."

The study was published in the December issue of the Journal of the American Dental Association.