Parents should use a tiny smear of fluoride toothpaste to brush baby teeth twice daily as soon as they erupt, instead of waiting until children are older, according to new guidelines by the American Dental Association.

That advice overturns the A.D.A.’s decades-old recommendation to start using a pea-size amount at 24 months. Once children are 3 to 6 years old, then the amount should be increased to a pea-size dollop, the updated guidelines say.

To fight the rising number of cavities in the very young, the dental group now advises getting a jump-start on prevention. However, they emphasize only the tiniest amount of fluoride toothpaste should be used to minimize the risk of mild discoloration, white spots or streaking of the teeth, a condition called fluorosis that is caused by ingesting fluoride toothpaste at a young age.

“We want to minimize the amount of fluoride consumption to reduce the risk of fluorosis while simultaneously adding a preventive tool for kids 2 and under that we haven’t recommended previously,” said Dr. Jonathan Shenkin, a spokesman for the A.D.A. and a pediatric dentist in Augusta, Me. Only a tiny amount of toothpaste should be smeared on the brush since some youngsters are likely to ingest some of the fluoride, he said.

The change comes after a systematic review of 17 studies published in The Journal of the American Dental Association this month. It concluded that scientific evidence, though limited in children under age 6 and more robust in older children, demonstrated that fluoride toothpaste is effective in controlling tooth decay, and that “the appropriate amount” should be used “by all children regardless of age.”

An early start is crucial, Dr. Shenkin said, because children with dental decay are at greater risk of developing cavities as adults. “By starting earlier, we can effectively reduce a lifetime of disease for a lot of kids.”

Dr. Man Wai Ng, the dentist in chief at Boston Children’s Hospital, applauded the new recommendation and said, “It’s a great thing for parents to know: ‘Use a tiny amount of fluoride, and brush two times a day to counter the effects of frequent snacking.’”

Most of the children she sees with tooth decay are using “a training toothpaste without fluoride,” she said.

The new A.D.A. guidelines stress that children should spit out toothpaste as soon as they are able, but not being able to spit does not preclude the use of a rice-grain-size bit of fluoride toothpaste.

Dr. Shenkin has had some 5-year-old patients “still not using a fluoride toothpaste because parents don’t think they can spit it out yet.” The goal is to have parents monitor the amount of toothpaste and to assist in brushing. Children generally cannot properly brush until they have the dexterity to tie their shoes.

The American Academy of Pediatric Dentistry, another trade group, has long recommended using just a smear of fluoride toothpaste for children younger than 2 who are at risk for cavities, including those who go to bed with a bottle at night or have a family history of cavities. The best predictor of developing cavities in the future is a history of cavities.

Some dentists already counsel parents of children under 2, especially those at high risk, to brush with fluoride.

Current labels on toothpaste advise parents of children under 2 to consult a dentist or physician. The labels are meant to direct parents to visit the dentist, Dr. Ng, said, but instead, “parents jump to the conclusion that they shouldn’t be using fluoride toothpaste because their young child isn’t able to spit it out yet.”

It’s unclear when and if the labels will change, or what will happen to fluoride-free training toothpastes, which currently are marketed as “safe if swallowed.”

Dr. Ng said she hopes that other organizations like the Centers for Disease Control and Prevention, which currently recommends that a pea-size amount of fluoride toothpaste be used starting at age 2, will “follow suit and be consistent with a single set of guidelines.” In that way, “the public will be less confused.”