Mental retardation may be caused by a problem with chromosome 21, which requires prompt medical attention. Abnormalities in the number or structure of fetal chromosome 21 can cause Down syndrome, which can lead to growth retardation, malformations and, in severe cases, mental retardation. Patients may also have special facial features such as protruding eyes, a thick tongue, and small outer ears. People with Down syndrome may have different degrees of mental retardation, which affects their ability to speak, remember, and think abstractly, and has a serious impact on their daily lives. Mental retardation may also be caused by other chromosomal problems. It is recommended to consult a doctor as soon as possible and choose the appropriate treatment.