This section reviews the clinical features of Down syndrome, the differences between diagnostic and screening tests, and the different types of maternal blood screening tests used to identify patients at higher risk for Down syndrome.
Second trimester maternal serum screening is used to identify patients, irrespective of their age, who are at increased risk for Down syndrome and spinal cord defects. This test is performed between 15 and 20 weeks of pregnancy. The Triple Marker test was initially introduced for screening, but has recently been replaced by the QUAD test. However, the state of California still mandates by law that the Triple Marker test be offered to all patients. In California the physician can offer the QUAD test, but only after offering the Triple Marker test. The QUAD test is better for identifying fetuses with Down syndrome in women less than 35 years of age.
Recently, first-trimester maternal serum screening, with or without ultrasound evaluation of the fetus, has been shown to be useful for identifying patients at increased risk for Down syndrome. This test is performed earlier (11 weeks to 13 weeks 6 days) than second-trimester maternal serum screening.
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