If a 3-week-old child does not speak, there are several reasons: first, the child is mentally retarded. If the child has severe mental retardation, such as congenital hypothyroidism that is not controlled by medication, or if the child has trisomy 21, the child may be severely mentally retarded, which may cause the child to not speak at the age of 3 weeks. Secondly, if the child has a hearing impairment, if the child has a serious hearing impairment or even deafness, this can also cause the child not to speak, and timely interventions should be given to the child, such as the application of cochlear implants or hearing aids.