| Most people associate having a child with Down | | | | "false-positives" (test states the patient has the |
| syndrome with older women. While it is true that | | | | condition when the patient really doesn't) and |
| women over 35 do have an increased risk of having | | | | "false-negatives" (patient has the condition but the |
| a child with Down syndrome, 80% of these children | | | | test states he/she doesn't). |
| are born to those women under age thirty-five. | | | | Maternal Serum Screening |
| In the United States, approximately 5,000 babies with | | | | The mother's blood is checked for three items: |
| Down syndrome are born every year. A woman's | | | | alpha-fetoprotein (AFP), unconjugated estriol (uE3) |
| chance of having another baby with Down syndrome | | | | and human chorionic gonadotropin (hCG). These three |
| is approximately 1 in 100. | | | | are independent measurements, and when taken |
| Prenatal Screening for Down Syndrome | | | | along with the maternal age (discussed below), can |
| Over the last 10 years, new technology has | | | | calculate the risk of having a baby with Down |
| improved the methods of detection of Down | | | | syndrome. |
| syndrome. While there are ways to diagnose Down | | | | A very important consideration in the screening test |
| syndrome by obtaining fetal tissue samples by | | | | is the age of the fetus (gestational age). The correct |
| amniocentesis or chorionic villus sampling, it would not | | | | analysis of the different components depends on |
| be appropriate to examine every pregnancy this way. | | | | knowing the gestational age precisely. The best way |
| Besides greatly increasing the cost of medical care, | | | | to determine that is by ultrasound. |
| these methods do carry a slight amount of risk to | | | | Test results are sometimes reported to doctors as |
| the fetus. | | | | "Multiples of the Median (MoM)." The "average" value |
| So screening tests have been developed to try to | | | | is therefore called 1.0 MoM. Down syndrome |
| identify those pregnancies at "high risk." These | | | | pregnancies have lower levels of AFP and estriol, so |
| pregnancies are then candidates for further diagnostic | | | | their levels would be less than 1.0 MOM.hCG in a |
| testing. | | | | Down syndrome pregnancy would be greater than |
| Screening Vs Diagnostic Test | | | | 1.0 MoM. |
| What is the difference between a screening test and | | | | Finally, the calculated risk is used to modify the risk |
| a diagnostic test? In diagnostic tests, a positive result | | | | already statistically calculated based on the mother's |
| very likely means the patient has the disease or | | | | age. We already know that as the mother's age |
| condition of concern. In screening tests, the goal is to | | | | advances, the risk of having a baby with Down |
| estimate the risk of the patient having the disease or | | | | syndrome increases. |
| condition. | | | | For example: Let's say the test results come back in |
| Diagnostic tests tend to be more expensive and | | | | the typical range for a pregnancy not associated with |
| require an elaborate procedure; screening tests are | | | | Down syndrome (that would be 1.0 MoM for all |
| quick and easy to do. However, screening tests have | | | | components). This result reduces the woman's risk of |
| more chances of being wrong: there are | | | | having a child with Down syndrome four-fold. |