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