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Genetics and Maternal Serum Screening:

Straightforward answers to
important questions

Maternal serum screening is designed to identify pregnant women who could benefit from prenatal testing for Down syndrome, spina bifida, and certain other birth defects.  These tests are done on a blood sample and pose no risk to the developing baby.

Why screen all pregnant women?
During a pregnancy, every couple is concerned about whether their baby will be healthy.  Fortunately, over 95% of babies do not have any significant health or learning problems.  Screening for two relatively common disorders, Down syndrome and spina bifida, early in pregnancy permits women at risk for these serious conditions to avail themselves of prenatal diagnosis in the first or second trimester of pregnancy.
Why focus on Down syndrome and spina bifida?
Down syndrome, which occurs in approximately 1 in 800 births, is one of the most common reasons for significant mental retardation.  Spina bifida, which occurs in about 1 in 1,000 births, is nearly always associated with serious medical and neurologic problems.  In most cases, there is no family history of these disorders.

Alpha-fetoprotein (AFP) is measured in maternal blood, and an elevation indicates an increased risk of a baby having spina bifida.  A low AFP value increases the risk that the baby has Down syndrome.  Beta-hCG (the hormone measured in blood and urine pregnancy tests) is elevated in pregnancies where the fetus has Down syndrome.  Other substances present in maternal blood can also be measured and help in estimating the risk of having a baby with Down syndrome.  Multiple marker serum screening involves measuring AFP and one or more additional substances.  When three substances are measured, it is sometimes referred to as a triple screen, and a quad screen involves measuring AFP, beta-hCG, unconjugated estriol, and inhibin-A.  Screening in the first trimester uses a combination of a measurement of the fetal nuchal translucency and analysis of a blood sample for PAPP-A and beta-hCG.  When results of first and second trimester screening are combined, this is known as sequential or integrated screening.  Obstetricians typically offer serum screening to all pregnant women.  High risk women can be offered newly available screening by analysis of cell-free fetal DNA that is present in a maternal blood sample.
How does a screening test differ from a diagnostic test?
Screening tests are designed to identify those individuals who might benefit from diagnostic testing.  Screening tests are usually less than 100% sensitive, and false positives occur.  Diagnostic tests are highly accurate and are designed to accurately predict whether or not there is a problem.
When is serum screening performed?
Multiple marker screening for Down syndrome and spina bifida is typically performed at 15-17 weeks gestation, although it can be performed at 14-22 weeks gestation, if necessary.  Screening in the first trimester typically occurs at 10-13 weeks gestation, usually in conjunction with ultrasound measurement of the fetal nuchal translucency and gestational age.
What if screening tests are normal?
Normal results of screening should be reassuring.  A negative result typically means that no further testing for Down syndrome or spina bifida will be necessary.  While screening tests for Down syndrome and spina bifida do not always identify an affected fetus, maternal serum screening detects over 85 percent of fetuses with spina bifida, and 80-90% of fetuses with Down syndrome.  Screening for Down syndrome using cell-free fetal DNA promises to identify as many as 99+% of fetuses with Down syndrome.

The decision to have screening for Down syndrome and spina bifida is a personal choice.  Obstetricians usually recommend that screening be considered, and they are able to answer questions about these studies.  It is up to each individual or couple to determine whether maternal serum screening is appropriate for them.

What if screening is abnormal?
Chromosome analysis on the fetus, either by amniocentesis or chorionic villus sampling (CVS), provides a definitive assessment of the fetal chromosome status.  Both prenatal diagnostic procedures carry small risks, primarily for miscarriage.  [The risk from amniocentesis is less than 0.5%, and the risk of CVS is no more than 1%.]  Accordingly, not all women with positive screening results will wish to pursue definitive prenatal diagnosis.  CVS can be performed at 10-14 weeks gestation, and amniocentesis can be performed at 15 weeks gestation and beyond.  A couple may be asked to consult a geneticist, who will obtain a detailed family and pregnancy history, review the results of the screening tests, and help the couple and the obstetrician decide how to proceed with further testing.

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