In the fourth month of pregnancy, women are usually required to undergo Down’s syndrome screening to determine the level of risk of the fetus having Down’s syndrome. In the vast majority of cases, the number of people with a critical risk and high risk of Down screening is relatively small, and only a very small percentage of women will have a critical risk of Down screening, a high risk of Down screening, and even fewer will have a high risk of Down screening. The reason is that only a small percentage of women may have a fetal abnormality or have a calculated abnormality and have a high risk of Down screening. There are also fewer people with a critical risk for Down screening because many women have a normal fetus after pregnancy, so there is no significant risk for testing for Down screening. However, many fetuses at critical risk are normal fetuses and only to further confirm the diagnosis, so pregnant women will be advised to have a non-invasive DNA test to rule out abnormalities.