Down syndrome is caused by chromosomal abnormalities. Normally, a person has 46 pairs of 23 chromosomes. Trisomy 21 is the 21st pair of chromosomes in a fetus that is one more than the normal 2, and is called 21-trisomy. It is also known as Down syndrome. The incidence of Down syndrome correlates with the age of the mother at conception. There is no effective treatment for Down syndrome. The best course of action is to terminate the pregnancy before the mother gives birth. This is also commonly known as prenatal diagnosis. In the past, it was thought that those >35 years of age were at high risk, and that the risk increased with the age of the mother. However, Down’s syndrome can also be seen in pregnant women of younger age. Down’s screening can narrow the scope of invasive prenatal diagnosis without missing pregnant women who may be carrying a child with Down’s syndrome, so it is recommended that every pregnant woman should be screened for Down’s syndrome as a precaution.