As the opioid epidemic sweeps through rural America, an ever-greater number of drug-dependent newborns are straining hospital neonatal units and draining precious medical resources.

The problem has grown more quickly than realized and shows no signs of abating, researchers reported on Monday. Their study, published in JAMA Pediatrics, concludes for the first time that the increase in drug-dependent newborns has been disproportionately larger in rural areas.

The rising rates are due largely to widening use of opioids among pregnant women, the researchers found.

From 2004 to 2013, the proportion of newborns born dependent on drugs increased nearly sevenfold in hospitals in rural counties, to 7.5 per 1,000 from 1.2 per 1,000. By contrast, the uptick among urban infants was nearly fourfold, to 4.8 per 1,000 from 1.4 per 1,000.