Oral cavity's structural variations are involved in the successful breastfeeding of the newborn infants. The tongue-tie has been shown to cause breastfeeding problems. This project investigates the oral structural properties of the newborn infants and the efficacy and safety of their treatment options in breastfeeding problems. The project includes two studies: 1. A prospective follow-up study that investigates the epidemiology and associations between oral structures and breastfeeding problems in term and preterm infants; 2. A randomised, controlled study that investigates the efficacy and safety of tongue-tie treatments. Follow-up visits are planned two to three months, one year, and five years after randomisation along with a 6 months' e-questionnaire for all study patients. The study provides information at the population level.