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 womans 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 babys neck. Babies with Down Syndrome and trisomy 18 tend to have NT measurements that are larger than those of babies without these conditions. Results of the blood and NT measurements are combined,
Sequential screening is comprised of a first trimester ultrasound of the nuchal translucency (NT) and blood test between 11 and 14 weeks. The preliminary results are available after completion of the first blood test. A second blood test, the Afp4, is obtained between 16 and 22 menstrual weeks, with the final results of Sequential screening available one week after Afp4 analysis. Sequential screening detects 90.4% of fetuses at risk for Down syndrome and 80% of fetuses at risk for neural tube defects. The advantage of the Sequential screening is that preliminary results are available during first trimester after the NT results and first trimester blood test have been analyzed.
This is similar to Integrated Screening, but results are shared with the patient after the first and second steps of the screening process. Approximately 91% of pregnancies affected with Down syndrome will present with an increased risk or a screen positive result on a Sequential screening test. What is a Neural Tube Defect? Very early in the pregnancy, the fetus forms a structure called the neural tube. This structure develops into the brain and spinal cord. Occasionally, the development of the neural tube can be interrupted, leading to a neural tube defect. A neural tube defect can prevent the brain and/or spinal cord from developing fully. Spina bifida and anencephaly are conditions that result from neural tube defects. The handicaps due to these conditions range from moderate to severe. What is Down Syndrome? Down Syndrome is a chromosome abnormality. Chromosomes contain the inherited information that instructs our bodies how to grow and develop. The correct number of chromosomes i