The cause of Down syndrome is an abnormality in the germ cells of one of the progenitors, which may be associated with the father or the mother, depending on the individual. Down syndrome is a chromosomal disorder characterized by trisomy of chromosome 21, which is caused by the presence of an extra chromosome 21 in the embryo’s somatic cells due to a nondisjunction of chromosome 21 in the germ cells of one of the progenitors (either the father or the mother) during meiosis. In some children, the disease can also be caused by a chromosome 21 abnormality that occurs during mitosis of a fertilized egg. For pregnant women who have given birth to a child with trisomy 21 and other high-risk pregnant women such as those of advanced age, amniotic fluid chromosome testing should be performed during pregnancy to prevent the birth of Down syndrome. If you have any other questions, it is recommended that you consult a medical professional for guidance.