Tess Merrell had breastfed three babies and never expected trouble with her fourth. But after a month of struggling with her newborn, she hired Melanie Henstrom for help. Henstrom, a lactation consultant, identified a culprit: The infant’s tongue was tethered to the bottom of her mouth. It was a common problem, she said, and could be fixed with a quick procedure at a dentist’s office. Sign up for The Morning newsletter from the New York Times “It was touted as this miracle cure,” said Merrell, a