Down syndrome is a condition caused by an abnormality of chromosome 21, which can be triggered by advanced maternal age, genetic factors and teratogenic substances. The cause of Down syndrome is the failure of chromosome 21 to segregate during meiosis, or mitosis after union into a fertilized egg, in either the father’s or mother’s germ cells, resulting in an extra chromosome 21. Maternal causes account for more than 90% of the cases. The risk of developing the disease is higher at advanced maternal age, especially when the pregnant woman is older than 35 years old. The prevalence is higher in children born to couples with Down syndrome. Exposure to teratogenic substances, such as radiation, benzene, and hepatitis B virus infection, may also cause chromosomal aberrations. When Down syndrome occurs, it is important to seek medical attention and follow the doctor’s instructions for appropriate treatment.