The medical advances that make young children less susceptible to mumps, polio and other diseases also mean that babies enter the world with a heavy vaccination schedule in store. The CDC suggests that parents bring their babies to the pediatrician for a total of 18 immunizations plus the flu vaccine before they even turn 9 months old.

But a new study of more than 97,700 children from Portland, Oregon, published in the journal Pediatrics, indicates that an increasing number of parents are straying from the CDC's vaccine plan.

The CDC schedule calls for five or more vaccines per visit at some early childhood appointments, but “shot-limiters” –- children whose parents attempt to control the number of vaccinations they receive –- are becoming more prevalent in Portland, the study says.

The research, which relied on vaccination records gathered through the Oregon ALERT Immunization Information System, found that over the three years between 2006 and 2009, there was a 7 percent increase in shot-limiting for children between 0 and 9 months of age. Whereas only 2.5 percent of the 97,711 children studied were classed as “consistent shot-limiters” – i.e., babies “having no more than 2 vaccine injections on all immunization visits from birth up to 9 months of age” -- in 2006, that percentage jumped to 9.5 in 2009. Children who did not receive any vaccines were not included in the study.

Parents with a more by-the-book approach to vaccines -- “non-limiters” and “episodic limiters” -- took their children for an average of 3.2 injections on each of around three doctor visits, while “consistent shot-limiters” saw the doctor more than four times in the same period, getting only 1-2 shots per visit.

“Shot-limiting” parents do not necessarily seek to limit the aggregate number of vaccines their children receive, but because of their insistence on spreading shots over so many appointments, these parents sometimes simply skip important vaccines, the study found.

The study’s authors named parental skepticism, fear of children’s pain, and “mistrust toward industries and governments associated with vaccination” among reasons why parents might postpone or avoid shots. They added that “media attention regarding vaccine safety issues likely contributed to the observed increase in shot-limiting in 2007.”

On its Vaccines & Immunizations site, the CDC recognizes that some of the diseases babies are immunized against are “becoming very rare” -– but says it’s still important for children to get the recommended shots:

Even if there are only a few cases of disease today, if we take away the protection given by vaccination, more and more people will be infected and will spread disease to others. Soon we will undo the progress we have made over the years.

Finally, the authors of the Pediatrics study note that ignoring the accepted vaccination schedule can be dangerous: “Delaying receipt of vaccines will unnecessarily increase the amount of time children are susceptible to vaccine-preventable diseases; there are known risks to the child but no known benefits associated with use of alternative schedules.”

Earlier on HuffPost: