In the first stage, an ultrasound scan is used to precisely determine the gestational age of the pregnancy (to ensure that testing is being performed at the proper time.) An ultrasound measurement of the baby’s neck called a nuchal translucency is taken (NT and Nasal Bone Sonographer must be certified), and a blood sample is drawn to measure the level of pregnancy associated plasma protein (PAPP-A) and total b-hCG.
After the evaluation of the data from the first stage, two risk groups are identified:
- High Risk Patients – Patients with a risk greater than or equal to 1 in 30 receive a screen positive result. The second stage of screening is skipped, and the patient is offered the option of further diagnostic testing.
- All Other Risks: Patients with a risk less than 1 in 30 proceed to the second stage of screening.
In the second stage, a second blood sample is taken to measure levels of four markers:
- Alpha-fetoprotein (AFP)
- Total beta-human chorionic gonadotropin (total ß-hCG)
- Unconjugated Estriol (uE3)
In pregnancies affected by Down syndrome, the levels of PAPP-A, AFP, and uE3 tend to be lower than normal while the levels of inhibin and total ß-hCG are elevated. The measurement from the NT may also be thicker than normal in an affected pregnancy.
In a pregnancy affected by ONTDs such as open spina bifida or anencephaly, the level of AFP tends to be elevated.
- What Does A Screen Negative Test Result Mean?
- If the risk of Down syndrome after the second stage of screening is lower than 1 in 270, then the result is considered a screen negative result for Down syndrome.Although a screen negative test result means that the patient is not at high risk for having a baby with Down syndrome, a screen negative result does not completely rule out the possibility of a pregnancy with Down syndrome.
- What Does A Screen Positive Result Mean?
- A screen positive result for the Sequential test means there is a risk of greater than or equal to 1 in 30 during in the first stage and 1 in 190 during the second stage for having a pregnancy affected with Down Syndrome. About 0.6% of patients in the first stage and 2.25% of patients in the second stage receive a screen positive result.
- Why Will Some Patients Have To Wait Until The Second Stage To Have A Risk Estimate?
- Patients determined to be at elevated risk are identified in the first stage and risk assessment ends for them. The remainder of patients will proceed to the second stage. These patients will receive results using information from the first and second stages.