Mental retardation in 3-week-old babies will be signaled by abnormal crying, poor mobility and delayed speech. 1. Abnormal crying: an important sign of mental retardation in 3-week-old babies is abnormal crying. Some babies will cry for a long time after being stimulated, but sometimes it takes repeated stimulation or continuous stimulation to cause crying. 2. Poor mobility: 3-week-old babies with mental retardation may also have poor development of mobility functions, which is mainly manifested in the child’s clumsy movements, such as walking, running, sitting and standing. 3. Language delay: 3-week-old babies with mental retardation may have language disorders, lack of rich vocabulary and unclear pronunciation. Usually a normal 3-year-old will use three words to form a simple sentence to express his inner thoughts, but children with mental retardation lack this ability. Parents of 3-week-old babies with these symptoms should seek prompt medical attention to identify the cause of the problem and treat it correctly.