First Trimester Screening

Premier Fetal Screening

A premier service for the screening for Down Syndrome by the Prenatal Consultants

Introduction

The vast majority of babies are normal. However all women 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. It occurs in 1 out of every 700 babies born. It causes learning difficulties and, in may instances, multiple medical problems.
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 on how to continue the pregnancy.
Other couples may choose not to be tested for Down Syndrome if they are willing to accept their risk of delivering a handicapped child.
An invasive diagnostic test is the only way to know definitely if the fetus has a chromosomal abnormality. However, such a test is associated with a risk of miscarriage (less than 1%).
Screening tests are available to better estimate the risk of Down Syndrome in each pregnancy. This may help couples decide if they want an invasive test.

Contemporary Down Syndrome Screening

Screening for Down Syndrome in each pregnancy can be achieved by the following methods:

Figure 1. Relationship of risk of Down Syndrome to maternal age

Mother's age. The risk of having a child with Down syndrome increases with the mother's age (see Figure 1). However if only women aged 35 years or older is offered an invasive test, Down Syndrome in younger women will not be detected.
Maternal serum screening where the results of a simple blood test done between 15-20 weeks of pregnancy is combined with the mother's age to estimate the risk of Down Syndrome.
The 18-20 week scan where the presence or absence of structural abnormalities or soft tissue markers can be used to better predict the risk of Down Syndrome.

Premier Fetal Screening

Premier Fetal Screening offers three packages (see Table in Summary Points) that increase the detection rate of Down Syndrome while minimizing invasive testing.

All packages include an ultrasound scan at 11-14 weeks. This can be carried out over the abdomen. However in some cases, the ultrasound may be performed through the vagina if the fetus is not in the right position.

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. These 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.

The risk of Down Syndrome may be evaluated based on the age of the mother, the nuchal translucency with or without any blood test (see Figure 4).

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.

Package A consists only of an ultrasound scan. A blood test taken at the time of the ultrasound scan (Package B) or later at 15-20 weeks (Package C) may further increase the accuracy of the screening test (see Figure 5).

Figure 5. Detection rates for Down Syndrome for various screening strategies based on models).

Summary Points

Ultrasound scan of nuchal translucency (NT) at 11-14 weeks is the single most accurate test for the screening of Down Syndrome.

The packages are:

Package	Tests	Detection Rate	Results available on	Price
A	Ultrasound scan at 11-14 weeks	80-85%	Day of scan	$157.50
B	Ultrasound scan at 11-14 weeks Blood test at 11-14 weeks	90%	2 working days after blood test	$315.00
C	Ultrasound scan at 11-14 weeks Blood test at 15-20 weeks	90%	2 working days after blood test	$245.40

Note:

The blood test in Package B measures PAPP-A and free beta-hCG. The blood test in Package C measures alpha fetoprotein and beta-hCG.
Detection rate is estimated based on models where 5% of those screened would be labeled as high risk.

For older women, such tests may show low risk results (i.e. <1:300). These results may offer reassurance enough for most parents to avoid an invasive procedure such as amniocentesis.
For younger women, 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), the tests allow parents an opportunity to opt for an invasive procedure.
Most of the "high risk" results (i.e.> = 1:300) would show normal chromosomes of the baby when amniocentesis is done.
Packages A, B and C are screening tests that estimate the risk for Down Syndrome and are not diagnostic tests. Diagnostic tests are invasive in nature and are associated with small risks of miscarriage.
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 KK Hospital providing this service have been accredited by the Fetal Medicine Foundation.

Common Questions and Answers

What is Down Syndrome?
Down Syndrome is the most common cause of learning difficulties in newborn babies. It occurs in 1 out of every 700 babies. Anyone can have a baby with Down Syndrome, regardless of age, family history or ethnic background. However, a woman's risk of having a baby with Down Syndrome increases with age.

Down Syndrome is caused by having an extra copy of a particular chromosome (the part of cells that contain genetic material from our parents).

Individuals with Down Syndrome will have some degree of learning difficulty. They are also at greater risk of having some medical problems, including heart defects, abnormalities of the digestive system, hearing and eyesight problems, seizures, thyroid problems and childhood leukaemia. Life expectancy is around 40 to 60 years.

The severity of the condition varies from person to person. Prenatal testing can tell if a baby has Down Syndrome. However, it is not able to predict how severely an individual will be affected.

What happens during the 11-14 weeks scan?
A moderately-filled bladder is generally preferred before the scan.

The length of the fetus would be measured. It must measure between 45 to 84 mm for any risk estimation to be valid. If the length of the fetus is less than 45 mm, a repeat scan will rescheduled. If the length is more than 84 mm, an alternative method of screening for Down Syndrome will be discussed with you.

The nuchal translucency (NT) is measured at the back of the baby's neck. If the measurement taken is larger than expected, this means that there is an increased risk that the baby could have Down Syndrome or other chromosomal abnormality. However, most babies with a large NT measurement are born without Down Syndrome.

The computer software will use your date of birth and your gestational date (how many weeks pregnant you are) to calculate your age-related risk. This is then combined with the NT measurement to calculate the overall risk of Down Syndrome.

Is detection of nasal bone useful?
Research has shown that two-thirds of fetuses with Down Syndrome and only 1% of fetuses with normal chromosomes have absent nasal bone on the 11-14 week ultrasound scan. Hence the presence of nasal bone on ultrasound scan in reassuring. The presence of a nasal bone would further reduce the risk of Down Syndrome, while its absence would further increase that risk. If the nasal bone is absent, we suggest that a re-scan at 16 weeks be arranged to re-check and to measure the nasal bone if this is present.

How should I interpret the results of the Down Syndrome screening test?
As these tests are screening tests, the results will be given to you and your partner in the form of an estimated risk, based on the best available current scientific data.

For example, if you are informed that your baby's risk of Down Syndrome is 1 in 100, it means that among 100 similar pregnancies with the same measurement of the nuchal translucency, one pregnancy will eventually result in the birth of a baby with Down Syndrome.

What other tests are available?
If you are at increased risk (usually more than 1 in 300) of having a child with Down Syndrome, you will be offered another diagnostic test such as chorionic villus sampling or amniocentesis. This test will give you a definite diagnosis of Down Syndrome or other major chromosome abnormalities. However these tests carry a small risk (about 1%) of a miscarriage.

What happens if Down Syndrome is diagnosed in my baby?
Although some medical complications resulting from Down Syndrome can be treated, there is no available specific treatment for any chromosomal abnormality. Some patients carrying Down Syndrome fetuses may choose to carry on while others may choose to abort.