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The Test Sample
What is being tested?
Maternal serum screening, or the multiple marker test, measures several substances in the blood of a pregnant woman in the second trimester of pregnancy. The results can be used to determine the risk that the baby has a chromosome disorder or neural tube defect such as spina bifida.
These substances include the following, listed below. When the first three substances are measured together, it is called a triple screen. When the fourth substance, inhibin A, is added, it is called a quad screen.
- Alpha-fetoprotein (AFP) is a protein produced by fetal tissue. During development, AFP levels in fetal blood and amniotic fluid rise until about 12 weeks, and then levels gradually fall until birth. Some AFP crosses the placenta into the mother's blood. With certain problems affecting a baby, an increased amount of AFP leaks into amniotic fluid and then into the mother's blood. As such, an AFP test is used to screen for neural tube defects such as spina bifida. It may be performed as part of the triple or quad screen. It may be performed by itself when risk for chromosome disorders (Down syndrome (trisomy 21) and Edwards syndrome (trisomy 18)) have already been assessed using first trimester Down syndrome screening or cell-free fetal DNA testing. According to the American College of Obstetricians and Gynecologists (ACOG), the AFP test detects neural tube defects in 85% of the cases.
- Human chorionic gonadotropin (hCG) is a hormone produced by the placenta. Levels in maternal blood rise for the first trimester of pregnancy and then decrease during the remainder of the pregnancy. In pregnancies where the fetus is carrying the chromosomal defect that results in Down syndrome, hCG tends to be high whereas in pregnancies where the fetus has the chromosomal defect that results in Edwards syndrome, hCG tends to be low.
- Unconjugated estriol (uE3) is a form of estrogen that is produced by the fetus through metabolism. This process involves the liver, adrenals, and the placenta. Some of the unconjugated estriol crosses the placenta and can be measured in the mother's blood. Levels rise around the 8th week and continue to increase until shortly before delivery. In pregnancies where the fetus has Down syndrome or Edwards syndrome, uE3 tends to be low.
- Inhibin A is a hormone produced by the placenta. Inhibin is a dimer (has two parts) and is sometimes referred to as DIA or dimeric inhibin A. Levels in maternal blood decrease slightly from 14 to 17 weeks gestation and then rise again. Levels tend to be higher in pregnancies where the fetus has Down syndrome.
Including the fourth marker, inhibin A, increases both the sensitivity and specificity of the screen for Down syndrome. According to ACOG, the triple screen detects Down syndrome in 69% of the cases while the quad screen detects it in 81% of the cases.
How is the sample collected for testing?
Blood is drawn from a vein in the woman's arm.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.