Q: Is there an early screening test for Down Syndrome?
A: You can screen for Down syndrome by ultrasound as early as 11-12 weeks. Screening is done with an ultrasound (sonogram) examination and two blood tests, and it can detect up to 90% of Down syndrome fetuses.
- Ultrasound: Fetal Nuchal Thickness
Detection of pregnancies with Down syndrome can be improved with ultrasound measurement of fetal nuchal thickness or translucency 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%.
- Blood Markers
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 a 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.