What is sequential screening?
Sequential screening is a two-stage screening procedure offered during pregnancy to identify women who are at increased risk of having a baby with Down Syndrome.
It also permits screening for open neural tube defects, such as open spina bifida and the identification of pregnancies at high risk for trisomy 18.
The first stage of sequential screening is offered between the 10th and 13th weeks of pregnancy and requires a blood sample and an ultrasound examination. The blood sample is used to measure two proteins that are found in a pregnant woman’s blood: pregnancy-associated plasma protein A (PAPP-A) and human chorionic gonadotropin (hCG).
An ultrasound exam of the baby is performed to measure the nuchal translucency (NT). Nuchal translucency refers to a collection of fluid in the back of the baby’s neck. Babies with Down Syndrome and trisomy 18 tend to have larger NT measurements than those of babies without these conditions.
Results of the blood and NT measurements are combined and a risk for Down Syndrome and trisomy 18 is determined. If a baby is found to be at very high risk for either Down Syndrome or trisomy 18, a patient is offered diagnostic testing. Most women (more than 99 percent) will not be in this very high-risk group and proceed to the second stage of sequential screening.
Stage 2 of sequential screening
The second stage of sequential screening is offered between the 15th and 21st weeks of pregnancy and requires a blood sample to measure four substances found in a pregnant woman’s blood: alpha-fetoprotein (AFP), hCG, unconjugated estriol (uE3) and dimeric inhibin A (DIA).
The NT measurement and the measurements from both blood samples are then combined with information about the patient such as age and weight, to determine the baby’s final risk for having Down Syndrome. The AFP measurement is used to screen for open neural tube defects and the combination of the different markers may identify babies at increased risk for trisomy 18.