(NYTIMES) - Increasing numbers of infants in rural areas in the United States are born dependent on drugs, largely as a consequence of their mothers' use of opioids, researchers reported on Monday (Dec 12).

From 2004 to 2013, the rate of newborns experiencing withdrawal increased sevenfold among infants born in hospitals in rural counties, to 7.5 per 1,000 from 1.2 per 1,000.

By contrast, in that decade the uptick among urban infants was nearly fourfold, to 4.8 per 1,000 from 1.4 per 1,000.

"The problem is accelerating in rural areas to a greater degree than in urban areas," said Dr Veeral Tolia, a neonatologist who works at Baylor University Medical Center who was not involved in the new report.

Using data from 2012 and 2013, a recent federal study found states like West Virginia, Maine and Vermont had particularly high rates of what is known as neonatal abstinence syndrome.

It includes such symptoms as irritability, breathing problems, seizures and difficulties taking a bottle or being breast-fed.

But this latest report, published in Jama Pediatrics, concludes that for the first time, the increase in drug-dependent newborns has been disproportionately larger in rural counties.

In the 1970s, withdrawal symptoms affected mostly babies of heroin-addicted mothers in cities such as Philadelphia and New York, Dr Tolia said, adding, "What this study shows is this has totally flipped."

Babies may be born with withdrawal symptoms from any number of drugs, including certain antidepressants or barbiturates, after prolonged use during pregnancy. But the new report found that rates of infant drug-dependency rose in tandem with maternal opioid use in particular.

Doctors frequently prescribe opioids to mothers-to-be to treat back pain or abdominal pain. Nearly 42 per cent of pregnant women in Utah on Medicaid were prescribed opioids, and roughly 35 per cent in Idaho, a 2014 study found.

Maternal opioid use was nearly 70 per cent higher in rural counties than urban ones, the new report found.

It also found that rural infants and their mothers were more likely to be poorer and to rely on public insurance like Medicaid, but it did not address other factors behind the increase in babies born dependent on opioids.

Women who are already addicted "may not be getting the help they need before they get pregnant," said Dr Nicole Villapiano, a pediatrician at C. S. Mott Children's Hospital at the University of Michigan and the lead author of the new study.

Alternatively, she suggested that rural mothers-to-be might not have access to medical assistance to treat opioid addiction. For many of them, the logistics can be difficult.

Methadone, a drug commonly used in treatment programmes, must be distributed at a clinic every day, which is "very challenging if you're talking about a rural community where there isn't much local health care," Dr Tolia said.

Pregnant women may also be treated with buprenorphine, available at certain pharmacies. In theory, Dr Tolia said, it could be more easily distributed to rural areas.

"Our solutions need to be focused on where the disease is happening in communities," he said. Once a baby arrives in a neonatal intensive care unit, "it's too late."

Dr Anne Johnston, an associate professor of paediatrics at the University of Vermont, argued that part of the reason for the increase in drug dependence among babies may be that more pregnant women were being treated for their addiction.

Their babies are being more closely monitored, she suggested, a change from years past.

"If you're somebody who is actively using heroin or OxyContin, and you don't tell anyone and you present to the hospital in labour, your baby may not have signs of withdrawal until days after birth," Dr Johnston said.

"And by that time, you're already out of the hospital."