Here's what happens: When you're at 15 to 21 weeks, your blood sample will be analyzed for the amount of four different "markers" that help identify women at risk for neural tube defects (NTDs), trisomy 18 (Edwards syndrome) and trisomy 21 (Down syndrome).
- The neural tube develops into the brain and spinal cord during prenatal fetal development. Closure of the neural tube occurs between the 17th and 30th days after conception, when a woman may not even know she is pregnant. When the development is incomplete, an open or closed defect may occur anywhere along the path. Spina bifida is a defect occurring along the spine; anencephaly is an open defect or absence of a major portion of the brain, skull and scalp.
- Trisomy 18 (Edwards syndrome) occurs in 1 in every 6,000 to 8,000 births and is associated with low birth weight, mental retardation, and cranial, cardiac and kidney malformations. Most infants affected with this condition die within the first year of life.
- Trisomy 21 (Down syndrome) is associated with mental retardation; malformation of the heart, gastrointestinal tract, eyes and ears; and early Alzheimer's disease. The overall risk of having an affected fetus is 1 in 800 to 1,000 live births. The second-trimester risk is 1 in 270 in women 35 to 40 years of age, and 1 in 100 in women older than 40. Although the risk for trisomy 21 increases with maternal age, an estimated 75 percent of affected fetuses are born to mothers who are younger than 35. Because of this risk, it is important to provide pregnant women younger than 35 with noninvasive screening for this chromosomal defect. This is the basis for offering the quad screen to all women in pregnancy.