NASHVILLE, Tenn. — Brittany Hudson was pregnant, addicted to painkillers and afraid of a Tennessee law that calls for the arrest of mothers of drug-dependent babies. She eventually gave birth without medical help, on the side of a road in the foothills of the Great Smoky Mountains. Hudson’s dilemma, doctors say, was one of many unintended consequences of the Tennessee Legislature’s decision in 2014 to become the first and only state with an explicit criminal offense for these addicted mothers. The law was meant to deter drug abuse by threatening mothers with up to a year behind bars, while allowing them to avoid jail and have their assault convictions removed if they got drug treatment.