First Trimester Screening
The vast majority of babies are normal. However all expectant mothers have a small risk (about 2%) of delivering a baby with a major physical and / or mental handicap. This may be due to a chromosomal abnormality, genetic disease or structural abnormality in the baby. The commonest chromosomal abnormality is Down Syndrome.
Down Syndrome occurs in 1 out of every 700 babies born. It causes learning difficulties and, in many instances, multiple medical problems. All pregnancies, regardless of a woman’s age, are at risk for Down Syndrome.
Some couples may choose to be tested for Down Syndrome as such information may help them prepare psychologically for the birth of a handicapped child or reach a decision about whether or not to continue with the pregnancy.
Other couples may choose not to be tested for Down Syndrome, especially if they have decided to keep their pregnancy regardless.
Although invasive diagnostic testing through Chorionic Villus Sampling/Amniocentesis is the only way to know definitely if the fetus has a chromosomal abnormality. Such procedures are associated with a risk of miscarriage (0.3%).
Screening tests are instead available to better estimate the risk of Down Syndrome in each pregnancy. The result of the screening test may help couples to decide if they want to pursue invasive testing.
Contemporary Down Syndrome Screening
Screening for Down Syndrome in each pregnancy can be achieved by the following methods:
- Mother's age. The risk of having a child with Down syndrome increases with the mother's age (see Figure 1). However, all pregnancies regardless of a woman’s age are at risk for Down Syndrome. It is therefore a standard practice to offer screening for Down Syndrome to all pregnant women regardless of age.
- Maternal serum screening is a simple blood test done between 15-20 weeks of pregnancy that combines the mother's age with three biochemical markers to estimate the risk of Down Syndrome.
- The 18-20 week scan helps to detect ultrasound findings such as structural abnormalities or markers that can be used to assess the risk of Down Syndrome as well as other chromosomal/non-chromosomal causes.
Figure 1. Relationship of risk of Down Syndrome to maternal age
First Trimester Screening
First trimester screening (FTS) combines together an ultrasound scan, a maternal blood test and the pregnant woman’s age to assess her risk of having a pregnancy affected by Down Syndrome.
The ultrasound scan is performed between 11 weeks and 13 weeks 6days. This can be carried out over the abdomen. However in some cases, the ultrasound may be performed through the vagina if the features of the fetus cannot be clearly assessed due to fetal positioning.
The ultrasound scan entails a careful examination of the nuchal translucency (see Figure 2). The presence or absence of nasal bone (see Figure 3) and other structural abnormalities will also be evaluated during the scan. Combined together, these findings will give better estimates of the risk of Down Syndrome.
Figure 2. A proper measurement of the nuchal translucency (seen between the two white + markings) based on a well-magnified view of the fetus.
Figure 3. The nasal bone (shown on white arrow) is seen as a white line beneath the skin covering the nose.
Figure 4. Thickened nuchal translucency (measurements within the 2 white + markings) and absence of nasal bone (white arrow pointing to where the nasal bone should be) in a baby with Down Syndrome.
Maternal blood test
The maternal blood test can be done between 9 weeks and 13weeks 6 days of pregnancy, and measures the level of two pregnancy hormones PAPP-A (pregnancy associated plasma protein A) and free beta-hCG (free beta-human chorionic gonadotropin).
The risk of Down Syndrome will then be evaluated based on the age of the mother, the findings of the ultrasound scan and blood test.
FTS is only a screening test that estimates the risk for Down Syndrome. It is not diagnostic in nature.
- For older mothers, such tests may show low risk results (i.e. ≤1:300). These results may offer enough reassurance to avoid an invasive procedure such as amniocentesis or chorionic villus sampling.
- For younger mothers, most of the tests would show low risk results and thus offer further reassurance. In instances where the tests show high risk (i.e.≥ = 1:300), testing through chorionic villus sampling/amniocentesis may be performed for a diagnostic result.
- Most "high risk" results (i.e.≥ = 1:300) would show normal chromosomes of the baby when amniocentesis is done.
The detection rate of Down Syndrome using the NT measurement is high only if undertaken by properly trained and accredited individuals. The Fetal Medicine Foundation (www.fetalmedicine.com) currently offers such an accreditation programme. All the Prenatal Consultants at KKH providing this service have been accredited by the Fetal Medicine Foundation.
KK Antenatal Diagnostic Centre (ADC)
KK Women’s and Children’s Hospital
Women’s Tower Level 2
100 Bukit Timah Road
Appointments: 6294 4050