The quad screen is a screening test used to detect women at increased risk for three types of problems in their pregnancies: open neural tube defects (such as spina bifida, an opening along the spine), trisomy 18, and Down syndrome (both of which are chromosome abnormalities).
The quad screen measures the mother's blood for four pregnancy-related chemicals - AFP, estriol, hCG, and inhibin A - which should fall within a normal range. Pregnancies affected by trisomy 18 are more likely to yield low levels of three of the four chemicals. The risk for trisomy 18 is calculated from the mother's age and the three blood levels.
Chromosomes are the inherited structures in the cells of our bodies. There are 46 chromosomes in each cell, arranged into 23 pairs. Chromosome abnormalities involving a missing or extra chromosome are not inherited or caused by an exposure during pregnancy. Instead, they result from a chance mistake in cell division at the time of conception. This error is a random event that can occur in anyone's pregnancy.
Individuals with trisomy 18 have an extra #18 chromosome, thus three rather than two copies of chromosome 18. Babies with trisomy 18 have severe mental retardation and serious birth defects, which usually cause miscarriage during pregnancy or death usually by one year of age.
Since an abnormal quad screen may indicate an increased risk for trisomy 18 or other abnormalities in the baby, you have the option for diagnostic testing by amniocentesis. Genetic counseling to review the results of the screening test in more detail and discuss option for testing is available.