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A surge in U.S infants born with symptoms of withdrawal from heroin or strong prescription painkillers is driven largely by rising drug use among women in rural areas, a new study found.

The study, led by University of Michigan researchers, estimates that about 21 percent of newborns in rural counties had withdrawal symptoms in 2013, up from 13 percent in 2004.

Newborns whose mothers use these drugs during pregnancy are at risk for seizures, excessive crying, problems with breathing, sleeping and feeding and other withdrawal symptoms. Treatment sometimes includes methadone and babies may need to be hospitalized for weeks or months.

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The study shows the rate for affected rural newborns climbed six-fold — from about 1 per 1,000 births in 2004 to almost 8 per 1,000 births in 2013. By contrast, in urban areas the rate climbed from 1 per 1,000 to 5 per 1,000 births. The rates correspond with women's use of opioid drugs during pregnancy. This includes use or misuse of oxycodone and other prescription opioid painkillers, and use of illegal narcotics.

To come up with their estimate, the researchers analyzed a dataset of U.S. hospital discharges compiled by the government's Agency for Healthcare Research and Quality. The results were published Monday in JAMA Pediatrics.

Earlier studies also found an increase nationwide, and other data show rural areas have been hard hit by the U.S. opioid epidemic.

The new study shows there's an urgent need for more programs to make it easier for women in rural areas to obtain prevention services and treatment for addiction, the researchers said.