| SCREENING FOR DOWN'S SYNDROME |
In the past, advanced maternal age or a previous Down’s syndrome pregnancy were the only ways of identifying a high risk group. The risk of having a baby with Down’s syndrome increases with the mother's age. For example the risk is about 1 in 910 at age 30, and 1 in 28 at age 45. A previous affected pregnancy increases the risk further, to about 1 in 200 at age 30 and 1 in 25 at age 45.
Age |
Risk: 1 in | Age |
Risk: 1 in | |
| 20 | 1529 | 33 | 575 | |
| 21 | 1508 | 34 | 474 | |
| 22 | 1481 | 35 | 384 | |
| 23 | 1447 | 36 | 307 | |
| 24 | 1404 | 37 | 242 | |
| 25 | 1351 | 38 | 189 | |
| 26 | 1286 | 39 | 146 | |
27 |
1209 | 40 | 112 | |
| 28 | 1119 | 41 | 86 | |
| 29 | 1019 | 42 | 65 | |
| 30 | 910 | 43 | 49 | |
| 31 | 797 | 44 | 37 | |
| 32 | 683 | 45 | 28 |
However, with this approach a large number of normal pregnancies and relatively few affected pregnancies were identified as high risk. Most babies with Down's syndrome are born to young women - about half are born to women under 30 - since most pregnancies are in this age group. A very small proportion of affected births occur in couples with a family history.
Now, a simple blood test or a special ultrasound examination can be used to screen more effectively. This involves measuring ‘markers’ which are either chemicals in the mother’s blood or structures seen on ultrasound.
Primark
Test
Biomark
Test
Beta
TripleTest
| MARKERS |
The level of each marker is typically either increased or reduced on average in a Down's syndrome pregnancy. The table shows a typical profile for the most important markers found so far.
Marker |
Profile† |
nuchal translucency (NT) |
+++ |
human chorionic gonadotropin (hCG) |
++ |
inhibin-A |
++ |
free-beta hCG |
++ |
alpha hCG |
+ |
alpha-fetoprotein (AFP) |
– |
unconjugated estriol (uE3) |
– |
pregnancy associated plasma protein A (PAPP-A) |
– – – |
† the number of + and - signs gives the increase or decrease in a typical affected pregnancy.
All are blood markers except for nuchal translucency, which is a temporary swelling of the fetal neck measurable by ultrasound.
The average levels for each of the markers change with gestational age. To quantify the extent of increase or decrease in marker level they are expressed as multiples of the normal median (MoMs) for the gestation. For example, 2.0 MoM means that the level is double that expected for the gestational age of the pregnancy.
| INTERPRETATION |
Although, on average, a Down’s syndrome pregnancy follows a typical profile there is a lot of variability and many are atypical. Equally, some unaffected pregnancies have a profile similar to Down's syndrome. When someone is screened we use a computer program to calculate how close their profile is to that of an affected pregnancy.
Taking the maternal age, family history and profile together our program calculates the risk of the pregnancy ending in the birth of a baby with Down's syndrome. If the risk exceeds 1 in 250 the result is regarded as screen positive, otherwise it is screen negative. We also report the actual risk which could be as low as 1 in 50,000 or as high as 1 in 10.
