Doctors typically detect Down syndrome in pregnancy with a variety of screening and diagnostic tests. The screening tests, which consist of an ultrasound and three separate blood tests, help to determine the likelihood of the fetus being born with Down syndrome, also known as trisomy 21. If the results of the screening tests point towards this outcome, several diagnostic tests can be performed to determine whether or not the fetus has this chromosomal abnormality. The most common tests are an amniocentesis, chorionic villus (CVS), and a percutaneous umbilical blood sample (PUBS).
Screening tests begin in the latter part of the first trimester and the beginning of the second. An ultrasound is used to view the amount of fluid behind the fetus’ neck; chromosomal abnormalities tend to cause more fluid to collect in this area. During this time, typically between 11 and 14 weeks gestation, a blood test is performed to determine the amount of two different proteins in the mother’s blood.
The second screening test is done between 15 and 20 weeks gestation. Another blood test is performed to determine the amount of four different substances in the mother’s blood; abnormal amounts of alpha fetoprotein, estriol, human chorionic gonadotropin (hCG), and inhibin A could indicate down syndrome in pregnancy. The final screening test is the triple screen, which is performed between 16 and 18 weeks gestation. This test monitors the level of three different pregnancy associated proteins and substances, determining whether the levels of these substances are normal.
If the screening tests used to detect down syndrome in pregnancy come back abnormal, this means that the fetus the woman is carrying is more likely to have trisomy 21. It does not mean the fetus will have this abnormality. At this point, per the request of the mother, more invasive testing is performed to diagnose down syndrome in pregnancy.
Between 9 and 14 weeks gestation, a CVS test can be performed. This is typically done for women who know early on in pregnancy that they are at a higher risk of carrying a child with down syndrome. The CVS takes a small piece of the placenta to analyze the fetus’ chromosomes.
An amniocentesis, which tests the amniotic fluid around the fetus, can be performed as early as 15 weeks gestation. The test allows doctors to look at the fetus’ chromosomes and determine whether or not it has down syndrome. While the test is relatively safe, it does carry a small risk of miscarriage.
A PUBS test can be performed any time after 18 weeks gestation. Doctors take blood from the fetus’ umbilical cord and analyze the chromosomes. This test tends to carry the highest risk of miscarriage, and is typically only used to diagnose down syndrome in pregnancy if other diagnostic testing is inconclusive.