More and more parents in Oregon are choosing alternative vaccination schedules for their children, a practice known as shot-limiting, according to a new study in the journal Pediatrics. Experts say the practice puts children and those around them at greater risk for disease.

Researchers from the Oregon Health Authority found that the proportion of consistent shot-limiters in the area increased from 2.5 percent in 2006 to 9.5 percent of the population in 2009.

Although the study didn't identify a specific reason for the numbers, the researchers said parents may delay or miss vaccinations because they don't want their children to be in pain, or because they don't want their kids to have too many shots all at once, or because they question whether vaccines are really necessary. The researchers said that delaying shots not only meant not getting vaccines on time, but that many shot-limiting children missed certain vaccines altogether.

"It's easy for a baby on this alternative schedule to fall behind and not catch up," said Steve Robison, the study's lead author. "And we found that these children fell behind and stayed behind."

Children typically get a lot of vaccines in the first years of life. The U.S. Centers for Disease Control and Prevention recommends more than two dozen shots from birth to age 6 to protect kids against 14 different diseases. Squeezing in all of those vaccines can mean that children get five or six shots in a single doctor's visit.

But in the study, researchers found that 4,500 of nearly 98,000 children in Portland consistently had two or fewer shots per visit in their first nine months, even though those children went to the doctor's office more often.

Although the study looked only at children in Portland, other research suggests that parents around the country favor delaying or skipping vaccines. But experts say the practice is risky.

"A delay in vaccinating your children increases the amount of time they are susceptible to a number of diseases," said Dr. Richard Besser, ABC News' chief health and medical editor. "This is a clear indication of where the anti-science movement is having an impact."

Dr. Paul Offit, chief of infectious diseases at Children's Hospital of Philadelphia, said it may be easy for parents to misunderstand the risks posed if their children skip vaccinations for diseases that have been very rare for decades. But he said doctors have an obligation to emphasize the importance of immunization to all their patients.

"That first-line responder needs to be far more passionate about vaccines and what it means not to get them. It's not a theoretical risk anymore," he said.

But there has been a recent resurgence of certain diseases that could be prevented by vaccines, and doctors say it highlights the dangers of missing these shots. In May, Washington declared an outbreak of whooping cough or pertussis for the first time in decades. California declared an outbreak in 2010 after 9,000 people came down with the disease and 10 died from it. Several states have reported increasing cases of whooping cough, measles and mumps.

The study can't say whether these outbreaks are linked to alternative vaccine schedules, and doesn't indicate if more of the shot-limiters got sick.

Robison said it's important for parents to understand that the immunization schedule should be followed for a child's protection.

"There is a reason for the schedule: to get a number of these vaccinations in early when children are most vulnerable to these diseases," he said.