“It’s really remarkable how little we know about the effect of prescription drugs and even nonprescription drugs on the fetus,” said Dr. Nora D. Volkow, director of the National Institute for Drug Abuse. “There are real roadblocks in terms of helping us advance the field.”

Dr. Mark L. Hudak, a neonatologist in Jacksonville, Fla., is helping to revise the pediatrics academy’s guidelines. “There are commonalities, but it’s not like you can go to a Web site that says, ‘This is what should be used by everyone,’ ” Dr. Hudak said. “No one knows what the best approach is.”

Within states, every hospital that delivers babies exposed to painkillers may have its own approach. Eastern Maine treats affected newborns with tiny doses of methadone, while Maine Medical Center in Portland uses morphine combined with phenobarbital, a barbiturate that prevents seizures. Some hospitals are also experimenting with clonidine, a mild sedative that can relieve withdrawal symptoms.

There is growing debate over treatment for pregnant women addicted to prescription drugs, in light of concerns over the effects on their babies. Many are slowly weaned from their dependence with methadone, the standard of care for decades. Methadone, when taken in prescribed doses, keeps a steady amount of opiate in the body, preventing withdrawal and drug cravings that occur when levels dip. But it, too, can be addictive and cause nagging side effects like drowsiness. And for addiction treatment, it can be obtained only at federally licensed clinics where most users have to report for a daily dose.

A growing number of addicts are instead taking buprenorphine, another drug used to treat addiction that some studies suggest staves off drug cravings as effectively as methadone but is less likely to cause withdrawal in newborns. In rural areas of the nation, where methadone clinics are few, buprenorphine is considered a promising alternative because it can be prescribed by primary care doctors and taken at home.

But buprenorphine also appears not to work for some addicts.

Still, a study published in December in The New England Journal of Medicine showed that babies whose mothers had taken buprenorphine required significantly less medication after birth and less time in the hospital than did babies whose mothers were treated with methadone. But researchers cautioned that exposure to buprenorphine in utero can still cause withdrawal symptoms and that further study was needed.

“We don’t want it misconstrued that buprenorphine is a miracle drug,” said Hendrée E. Jones, a Johns Hopkins University researcher and the study’s lead author.