In the United States, approximately 5,000 babies with Down syndrome are born every year. A woman's chance of having another baby with Down syndrome is approximately 1 in 100.

Prenatal Screening for Down Syndrome

Over the last 10 years, new technology has improved the methods of detection of Down syndrome. While there are ways to diagnose Down syndrome by obtaining fetal tissue samples by amniocentesis or chorionic villus sampling, it would not be appropriate to examine every pregnancy this way. Besides greatly increasing the cost of medical care, these methods do carry a slight amount of risk to the fetus.

So screening tests have been developed to try to identify those pregnancies at "high risk." These pregnancies are then candidates for further diagnostic testing.

Screening Vs Diagnostic Test

What is the difference between a screening test and a diagnostic test? In diagnostic tests, a positive result very likely means the patient has the disease or condition of concern. In screening tests, the goal is to estimate the risk of the patient having the disease or condition.

Diagnostic tests tend to be more expensive and require an elaborate procedure; screening tests are quick and easy to do. However, screening tests have more chances of being wrong: there are "false-positives" (test states the patient has the condition when the patient really doesn't) and "false-negatives" (patient has the condition but the test states he/she doesn't).
Maternal Serum Screening

The mother's blood is checked for three items: alpha-fetoprotein (AFP), unconjugated estriol (uE3) and human chorionic gonadotropin (hCG). These three are independent measurements, and when taken along with the maternal age (discussed below), can calculate the risk of having a baby with Down syndrome.

A very important consideration in the screening test is the age of the fetus (gestational age). The correct analysis of the different components depends on knowing the gestational age precisely. The best way to determine that is by ultrasound.

Test results are sometimes reported to doctors as "Multiples of the Median (MoM)." The "average" value is therefore called 1.0 MoM. Down syndrome pregnancies have lower levels of AFP and estriol, so their levels would be less than 1.0 MOM.

hCG in a Down syndrome pregnancy would be greater than 1.0 MoM.

Finally, the calculated risk is used to modify the risk already statistically calculated based on the mother's age. We already know that as the mother's age advances, the risk of having a baby with Down syndrome increases.

For example: Let's say the test results come back in the typical range for a pregnancy not associated with Down syndrome (that would be 1.0 MoM for all components). This result reduces the woman's risk of having a child with Down syndrome four-fold.