Newborns who do not gain weight at 18 days are mostly caused by genetic metabolic diseases, neonatal hyperthyroidism, pyloric hypertrophy, and improper feeding. 1. Inherited metabolic diseases: Newborns with inherited metabolic diseases may lead to no weight gain. 2. Neonatal hyperthyroidism: Neonatal hyperthyroidism may also lead to the above situation, and will also be accompanied by decreased appetite, vomiting, diarrhea, irritability and so on. 3. pyloric hypertrophy: newborn pyloric hypertrophy, there will be the phenomenon of thin, typical symptoms of projectile vomiting, and will affect the newborn’s daily amount of milk and mental state. 4. Improper feeding: when breast milk is not enough, not in time to add formula milk, resulting in improper feeding, newborns do not gain weight. Newborns 18 days weight did not grow, there may be other causes, such as neonatal hypothyroidism, etc., it is recommended to go to the hospital in a timely manner, improve the examination to determine the cause of the disease, under the guidance of the doctor to give targeted treatment or treatment.