How is the sequential screen performed?
The sequential screen combines measurements from two blood tests and a first trimester ultrasound to tell you more about your developing baby.
Part 1 of the test can only be performed between 11 and 13 weeks of pregnancy. You will have an ultrasound to measure the fluid-filled space at the back of the baby’s neck (called the nuchal translucency) and a blood test to measure certain placental hormones and proteins. The lab will combine the results of your ultrasound, blood work and your age to calculate your sequential screen risk. If you used an egg donor to become pregnant the lab will use the age of your egg donor. If the result for part 1 is low risk then you will return for part 2 – another blood test at 16-18 weeks of pregnancy. The final risk combines part 1 and part 2 results.
Sometimes a nuchal translucency measurement cannot be obtained on the ultrasound due to the fetal position. The blood test can still be performed, but the results will not be available until after the part 2 blood test is completed. This is called serum integrated screening.
How accurate is the sequential screen?
The part one detection rate for Down syndrome and trisomy 18 is approximately 80%. The final sequential screen has a 90% detection rate for Down syndrome and trisomy 18, along with an 80% detection rate for open neural tube defects.
The serum integrated screen has an 85% detection rate for Down syndrome, a 90% detection rate for trisomy 18, and an 80% detection rate for open neural tube defects.
It is important to understand that screening tests will not detect all cases of Down syndrome. Therefore, it is possible that a woman with a screen result that shows a low risk can still give birth to a child with Down syndrome. Screening tests can also give false positive results. A positive result (showing an increased risk) does not mean that your baby definitely has Down syndrome, but indicates that you may wish to have further testing. A genetic counselor will help you understand what your risk means and discuss other testing options, such as chorionic villus sampling (CVS), amniocentesis, or cell-free DNA screening.