The 3-week-old baby’s lack of weight gain may be related to malnutrition, micronutrient deficiency, illness and other conditions. 1. Malnutrition: If the 3-week-old baby has a long time partiality, picky eating and other bad habits, resulting in insufficient nutritional intake, time is easy to cause malnutrition, it will cause the phenomenon of weight does not grow. 2. lack of trace elements: the lack of trace elements can also lead to 3-week-old baby weight does not grow, such as the baby lack of zinc, iron, magnesium and so on. 3. Sickness: If the 3-week-old baby suffers from gastrointestinal diseases and other diseases will also appear the weight does not grow. 3 weeks old baby weight may also have other reasons, it is recommended to go to the hospital in a timely manner, improve the examination to clarify the cause of the disease, under the guidance of the doctor to give targeted treatment or treatment.