At women pregnancy when they are older than 35, the risk of chromosomal irregularities risk increases and  trisomia 21 is often seen.

Normally seen once every 700-800 (less than 1%) births, down syndrome is increasingly seen when the woman is getting older. Between the age of 40-45, the ratio is 1%-4%. For this reason to the women who have advanced maternal age  must be told the risk of chromosomal anomaly before pregnancy and prenatal diagnosis should be offered.