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.
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.
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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