There are three different blood tests that are offered as part of prenatal screening for birth defects. The blood tests look at proteins produced by the baby and measure the levels of these proteins in the mother’s blood. There are a total of six fetal proteins that can be tested. These tests identify babies with Down Syndrome; other genetic conditions such as Trisomy 18, and babies with a condition called an open neural tube defect. An open neural tube defect is also known as spina bifida and is a condition where the spinal cord/and or brain does not form correctly very early in the pregnancy.
The sequential integrated screen measures all six of the fetal proteins found in the mother’s blood. The first blood test is performed between 10 and 13 weeks 6 days. The second blood test is performed between 15 and 20 weeks. A nuchal translucency ultrasound is also performed between 11 weeks 2 days and 14 weeks 2 days. This test is the most thorough and will identify approximately 92 percent of babies with Down Syndrome.
The serum integrated screen is the blood testing of the six fetal proteins just as described for the sequential integrated screen (first blood test between 10 weeks and 13 weeks 6 days and second blood tests between 15 and 20 weeks). The serum integrated screen identifies approximately 88 percent of babies with Down Syndrome. A serum integrated screen is typically the choice in centers where nuchal translucency ultrasound is not available (this special first trimester ultrasound requires specific training).
The quad marker screen is actually the second blood test of the sequential (and serum) integrated screen, meaning it is performed between 15 and 20 weeks. It measures four of the fetal proteins. A quad marker screen is typically offered when a women starts prenatal care too late to receive either the serum integrated screen or the sequential integrated screen. It has a lower detection rate for Down Syndrome and other birth defects than the other two tests, but still detects 79 percent of pregnancies with Down Syndrome.