Antenatal features of Down syndrome
Antenatal screening of Down syndrome (and other less common aneuploidies) should be available as a routine component of standard antenatal care. It allows families to either adjust to the idea of having a child with the condition, or to consider termination of pregnancy.
For a general description of Down syndrome and its postnatal manifestations, please refer to the parent article on Down syndrome.
Maternal age
There is a strong association between the incidence of Down syndrome and maternal age. Background risk based on maternal age is incorporated into both the serum screening based risk calculations, and into the calculation of increased risk in the presence of a soft marker in 2nd trimester (see below).
Specific data to follow
Maternal serum screening
Please refer to the antenatal screening for a general discussion of the avenues of screening and diagnosis in the antenatal period.
1st trimester
Combined serum screening has an approximately 85% detection rate, with 5% false positive rate 8.
Serological markers : MSS1
- maternal beta HCG :
- higher than chromosomally normal fetuses
- difference increases with gestation
- PAPP-A :
- lower than chromosomally normal fetuses
- difference decreases with gestation : therefore not commonly used as a second trimester test
2nd trimester (quadruple scan)
Detection rate for trisomy 21 is at approximately 80% with a false positive rate of ~ 5% 8.
Serological markers : MSS2
- maternal free beta HCG : higher than chromosomally normal fetuses
- inhibin A : higher than chromosomally normal fetuses
- AFP : lower than chromosomally normal fetuses
- unconjugated oestriol (uE3) : lower than chromosomally normal fetuses
Ultrasound
First trimester
Nuchal translucency : thickness depends on the size of the fetus (CRL), but in general it is considered abnormal if > 3 mm.
There is recent evidence that the inclusion of nasal bone measurement improves the specificity of 1st trimester data 5. This is not in universal practice at the time of writing.
Second trimester
Approximately 30% of babies with Down syndrome have detectable abnormalities on the mid-trimester ultrasound 1.
Soft markers
Soft markers are sonographic findings that do not in themselves cause any adverse outcomes. However, they are seen more frequently in fetuses with an abnormality. This article addresses the soft markers that are specific to Down syndrome. For a general discussion, please refer to the article on soft markers.
In the presence of a soft marker, risk of Down syndrome is recalculated as:
new risk = baseline risk x likelihood ratio (LR)
-
nuchal fold thickness > 6 mm
- likelihood ratio : 17 1
-
hypoplastic nasal bone
- has emerged in recent years as a particularly strong marker.
- absence of a nasal bone has a likelihood ratio of 831(!)
-
echogenic intracardiac focus
- likelihood ratio : 1.5 1
-
echogenic bowel
- likelihood ratio : 6.1
-
shortened humerus
- likelihood ratio : 7.5
-
shortened femur
- likelihood ratio : 2.7
-
single umbilical artery
- only considered a soft marker if other abnormalities are present. In an otherwise normal 2nd trimester ultrasound it is NOT a marker of aneuploidy.
-
mild pyelectasis
- weak association with aneuploidy
Structural abnormalities
The following may be present in association with Down syndrome.
- cardiac defects
- abdominal
- duodenal atresia
- esophageal atresia
- omphalocele (more common with trisomy 18 9)
- central nervous system
- cranio-facial/ calvarial
- other
Adjunct features
Other features that may be present, but are neither a structural abnormality, or a validated soft marker 8
- hypoplastic 5th digit
- wide iliac angle
- shortened frontothalamic distance
- short fetal ear length
- brachydactyly
Some fetuses can develop transient abnormal myelopoiesis (TAM) particularly towards the 3rd trimester and can then develop fetal hepatomegaly 11.
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