Studies of baby animals have long suggested that going under anesthesia can have some harmful effects on a developing brain. Now some scientists want to find out whether those same drugs may pose subtle risks for human babies and toddlers.

It's a balancing act: Doctors don't want to unnecessarily frighten parents whose tots need general anesthesia for crucial surgery. There's no clear evidence of side effects, such as learning or attention impairment, in youngsters.

Wednesday, an anesthesia research group, partnered with the Food and Drug Administration, said it's time for a large study of children younger than 3 to settle the question.

Meanwhile, "surgeons, anesthesiologists and parents should consider carefully how urgently surgery is needed, particularly in children under 3 years of age," concluded a report in The New England Journal of Medicine co-authored by the FDA's current and former anesthesia chiefs along with doctors in the SmartTots research partnership.

Already, in the vast majority of cases, children that young only undergo surgery if it is medically necessary, not elective. Some operations, such as to correct birth defects, have better outcomes at earlier ages, surgeons recently told the FDA.

"Millions of kids safely undergo anesthesia," stressed SmartTots co-author Dr. Beverley Orser, a professor of anesthesia at the University of Toronto.

Those drugs have been used for decades so any big risk would have been spotted by now, she said. But with animal studies raising the possibility of subtle effects on behavior or learning, "we have to sort this out," Orser added.

At issue are drugs used for general anesthesia and sedation, not local anesthetics. Questions began years ago when experiments in newborn rats showed such drugs could kill certain brain cells. The FDA formed a partnership with the International Anesthesia Research Society, called SmartTots, to better study the issue. In 2012, SmartTots, the FDA and the American Academy of Pediatrics issued a public caution about the uncertainty.

Wednesday's report says studies in a range of species, including baby monkeys, have found similar neurotoxic effects. Do they have lasting consequences? In some cases, the animals showed later impairments in memory and attention. Importantly, the cumulative dose mattered.

That doesn't automatically translate to risk for humans. A few studies have tracked youngsters who underwent anesthesia earlier in life. Some haven't found problems, but one found those who received anesthesia multiple times had a higher risk of learning disabilities. Yet those kinds of studies can't tell if any problem was due to a drug, the stress of surgery or the original medical condition, Orser noted.

The proposed next step: A large, multihospital study that would assign youngsters now undergoing surgery to different kinds of anesthesia, and compare their outcomes.

What's the message for parents? Occasionally, some ask about this issue, and doctors must help them weigh an unknown risk with the fact that "your child needs to have surgery, and they do need an anesthetic," said Dr. Allison Kinder Ross, chief of pediatric anesthesia at Duke University Medical Center, who wasn't involved with Wednesday's report.

But anesthesia is used with some nonsurgical procedures, too, such as to keep tots still during MRI scans - and Ross said that's an area to try alternatives. For example, Duke has had parents or nurses crawl inside scanners to hold a child.

"Under 3 is certainly more challenging, but it's doable for a lot of kids," Ross said.

Last June, SmartTots convened doctors' groups to update its earlier cautionary statement, and a draft now being considered suggests postponing surgeries or other medical procedures "that could reasonably be delayed" in children under 3. It's not clear whether that language will be adopted.