How is it used?Maternal serum screening is used in the second trimester of pregnancy to help evaluate the risk that a fetus has certain abnormalities, including Down syndrome (trisomy 21) or Edwards syndrome (trisomy 18), or neural tube defects such as or a condition called . Markers are often combined into a triple or quad screen because their power lies in their use together. A mathematical calculation involving the levels of these substances (AFP, hCG, unconjugated estriol, and, sometimes, inhibin A) and considerations of maternal age, family history, weight, race, and diabetic status are used to determine a numeric risk for Down syndrome and for a few other chromosomal abnormalities, such as Edwards syndrome, in the fetus. This risk is compared with an established cut-off. If the risk is higher than the cut-off value, then it is considered a positive screening test and increased risk exists for carrying a fetus with one of these abnormalities. An AFP test may be performed by itself and not as part of a triple or quad screen, especially when First trimester Down syndrome screening has already been used to assess the risk for chromosomal abnormalities. The AFP is used to help determine the risk of neural tube defects. When is it ordered?The test is usually ordered between the 15th and 20th weeks of pregnancy. What does the test result mean?The interpretation of a test result should be provided by a or clinician who can explain the meaning of the results and offer choices about follow up. It is important to remember that screening tests are not diagnostic of a fetal abnormality; they indicate a normal or increased risk. Of all women who have positive screening results, only a very small number of them have babies who actually have a neural tube defect or abnormality. In pregnancies where the fetus is carrying the chromosomal defect that results in Down syndrome (trisomy 21), the levels of AFP and unconjugated estriol tend to be low and hCG and inhibin A levels high. In pregnancies where the fetus has Edwards syndrome (trisomy 18), unconjugated estriol and hCG levels are low and AFP levels can be variable. A baby with an open neural tube defect has an opening in its spine, head, or abdominal wall that allows higher-than-usual amounts of AFP to pass into the mother's blood. The other markers are not used in the evaluation of risk for carrying a fetus with a neural tube defect. Second Trimester Maternal Serum Screening Analyte PatternsIncreased risk for: | hCG | uE3 | AFP | Inhibin A | Open spina bifida | Normal | Normal | High | Not applicable | Anencephaly | Low | Low | High | Not applicable | Down syndrome | High | Low | Low | High | Edwards syndrome | Low | Low | Low | Not applicable | If a screen is positive, tests that are more definitive are needed to determine and confirm a diagnosis. These include high-resolution ultrasound and perhaps amniocentesis followed by chromosome analysis. These follow-up tests are used to help women and their doctors make decisions about the management of their pregnancies. Keep in mind that not all fetal abnormalities will give positive results on this screening. Is there anything else I should know?The test result is very dependent on accurate determination of the age of the fetus. If the gestational age of the fetus has not been accurately determined, the results may be either falsely high or low. In multiple gestation pregnancies, calculation of the risk of Down syndrome or Edwards syndrome is difficult. For twin pregnancies, a "pseudorisk" can be calculated comparing results to normal results in other twin pregnancies. Evaluation of the risk of open neural tube defects in twin pregnancies can be determined, although it is not as effective as in singleton pregnancies. For higher gestation pregnancies, risk cannot be calculated from these tests. If you have had a first trimester Down syndrome screen, then second trimester maternal serum screening is typically not performed because the risks for Down syndrome and Edwards syndrome have already been assessed. However, if you and your health practitioner wish to use the results of both first and second trimester screening to assess the risk of chromosome abnormalities, then integrated or sequential screening may be employed. For more on these, see Common Questions #5.
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