Dentists and lactation consultants around the country are pushing “tongue-tie releases” on new mothers struggling to breastfeed.

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.

Ms. 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.

“It was touted as this miracle cure,” said Ms. Merrell, a high school soccer coach in Boise, Idaho.

Ms. Henstrom recommended a dentist, who in December 2017 cut under the baby’s tongue with a laser. Within days, the infant, Eleanor, was refusing to eat and had become dangerously dehydrated, medical records show. She spent her first Christmas on a feeding tube.