Q: Is there an early screening test for Down syndrome?
A: There are several tests that can be done to screen for Down syndrome at the end of the first trimester:
- Cell-free fetal DNA testing can be done from the mother's blood as early as 9 weeks of the pregnancy.
- A sonogram measuring the fetus' neck in combination with two hormones in the mother's blood can be done around 11-13 weeks of the pregnancy.
Ultrasound: Fetal nuchal thickness
Detection of pregnancies with Down syndrome can be improved with ultrasound measurement of fetal nuchal thickness or translucency in combination with measurement of free beta hCG and PAPP-A in the mother's blood. This has been shown to increase detection for Down syndrome to over 80%.
Two blood markers, free beta hCG and PAPP-A (Pregnancy Associated Plasma Protein-A) can be useful indicators of Down syndrome or other types of fetal chromosome abnormality in the first trimester. In Down syndrome pregnancies, free beta hCG levels tend to be higher than normal, while for PAPP-A levels tend to be lower than normal. Measurements of free beta hCG and PAPP-A can be used to estimate the risk or probability that the fetus has Down syndrome and this risk can be used to modify the maternal age risk. Choosing, for example, a cut-off risk of 1:250 to define a high-risk group, 5% of women will be considered high-risk and about 60% of Down syndrome pregnancies will be identified within this group.
Cell-free fetal DNA testing
Cell-free fetal DNA testing is a new screening test where a sample of the woman's blood is taken after 10 weeks of the pregnancy. The cell-free DNA test is a "non-invasive" test which requires only a blood test. The cell-free DNA test is a screening test, not a diagnostic test. If provides risks for certain conditions but does not make the definite diagnosis
The cell-free DNA test indicates if a woman is at increased risk of having a fetus with Down syndrome (trisomy 21), Edward syndrome (trisomy 18) and Patau syndrome (trisomy 13). The test measures the relative amount of free fetal DNA in the mother's blood. The test determines the chance that the fetus has Down syndrome, Edward syndrome or Patau syndrome based on the relative amount of DNA from chromosomes 21, 18 and 13. usually additional tests are required like a chorionic villi sampling or CVS or an amniocentesis to determine the actual diagnosis.
The results of the cfDNA test are usually available within two weeks of the blood draw.
Cell-free fetal DNA testing is thought to detect greater than 99 percent of all Down syndrome pregnancies and greater than 98 percent of all trisomy 18 pregnancies. It detects about 65 percent of all trisomy 13 pregnancies.
Cell-free fetal DNA testing is not designed for use in women with multiple pregnancies such as twins, or in women with egg donor pregnancies.
Results of the cell-free DNA test
A cell-free DNA test is a screening test. You usually receive a percent (%) risk for the condition, not the absolute diagnosis. To make a diagnosis, you need additional tests such as a CVS or amniocentesis.