Fetal tissues have been sampled conventionally from the amniotic fluid (by amniocentesis)or placenta bed (by chorionic villus sampling CVS). The ability to identify, amplify and re-constitute genetic information using fetal DNA fragment from the mother's blood is a seismic paradigm shift.
Detecting if Down's Syndrome is present in a pregnancy and preventing its birth may be the only way to reduce the “burden” of Down's Syndrome. This burden is borne by not just the person with Down's Syndrome, but also the parents and the society at large. The decision whether to have early detection rests on the parents (especially the mother). There are pregnant women who do not wish to find out if the unborn baby has Down's Syndrome.
Confirming if Down's Syndrome is present in a pregnancy is by using invasive techniques like chorionic villus sampling and amniocentesis to obtain component cells from the unborn baby for genetic analysis. These diagnostic tests carry certain risk of miscarriage and therefore are not offered to all women.
In recent years, a group of screening tests have been developed to pick up women who are at high risk of carrying a Down's Syndrome baby. These screening tests do not have risk of miscarriage, and although they do not definitely tell us if the baby has Down's Syndorme, they indicate if the chance of the baby being one is high. When the screening test return a high risk result, the mother will be offered the confirmatory test. It is vital to understand that the screening tests cannot exclude if the baby definitely has or definitely has not Down's Syndrome. The sequential use of screening and confirmatory tests in high risk result is the current standard practice, and it is devised because there is no available risk-free diagnostic test at the moment.
A low risk result means exactly that the baby is unlikely to have DS, it does not mean the baby has no risk of having Down's Syndrome. Some risk is still present because the screening tests are not perfect. Some babies with Down's Syndrome are not detected because of natural variations affecting how they express their characteristics when the screening tests are performed. About 25% of babies with Down's Syndrome are not detected by screening tests.
About 1:30 (3%) of women who are screened returned with a high-risk result, and diagnostic test will be advised. Having a high-risk result does not imply that the baby definitely has DS. It means that the risk is significant to further evaluate. Appreciating that the risk is higher than 1:200, majority of those who go for the confirmatory test learn that their baby does not have Down's Syndrome.