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Down Syndrome - Should all pregnant women undergo First Trimester Screening?

Dr Tan Wei Ching, MBBS, MRCOG, RCR/RCOG Diploma in Adv Obst Ultrasound
Associate Consultant, Department of O & G,
Singapore General Hospital

Introduction

Most babies are normal. However, all women, regardless of age, will have a small risk of delivering a baby with a major physical and/or mental handicap. This may be due to chromosomal or structural abnormalities or genetic diseases in the baby. The commonest chromosomal abnormality is Down Syndrome. It occurs 1 in every 700 babies and can cause learning difficulties and multiple medical disorders.

Some couples may choose to screen their pregnancy for Down Syndrome. This provides the option of interrupting an affected pregnancy or help to prepare the couple psychologically for the birth of a handicapped child. Screening for Down Syndrome is not compulsory. Some couples may choose not to have their pregnancy screened for Down Syndrome if they accept the background risk of having an affected child.

Screening tests refine the risk of Down Syndrome in each pregnancy. Screening tests do not provide a diagnosis and do not pose a risk to the mother or fetus. Based on the screening results, pregnancies are categorised into high or low risk for Down Syndrome. An invasive test (like an amniocentesis) provides a diagnosis. Patients with a high risk result can opt for this test which carries a 1% risk of miscarriage.

Down Syndrom w maternal ageCurrent screening programmes for Down Syndrome in Singapore

The risk of having a child with Down Syndrome increases with the mother's age (Fig. 1). All pregnant mothers are offered the screening tests while oder mothers (> 35 years old) have the option of selecting the screening test or the invasive test, like an amniocentesis.

The Second Trimester Maternal Serum Screening (MSS) is performed between 16-22 weeks and the mother's blood is taken to measure the levels of beta-hCG, alpha-fetoprotein and estradiol to estimate the risk of Down Syndrome. 5% of the population screened will have a high risk result and will be offered an invasive test. This test detects 65% of babies with Down Syndrome.

The First Trimester Screening (FTS) is a more accurate way of estimating the risk of the pregnancy being affected by Down Syndrome and is performed between 11-14 weeks. The FTS calculates a composite risk based on maternal age, the Nuchal Translucency measurement (Fig. 2 & 3) and levels of beta-hCG and PAPPA in maternal blood. This constitutes the Combined Test which has the highest detection rate (90%) for Down Syndrome among the tests available in Singapore (Table 1).

 Nuchal Translucency  Thicken Nuchal Translucency
Figure 2. Nuchal translucency (seen between
the two white markings) of the fetus
Figure 3. Thickened nuchal translucency
(measurements within the 2 white + markings) in
a baby with Down Syndrome

Down Syndrome Detection Rates

Besides having a higher detection rate than the 2nd Trimester Maternal Serum Screening tests (MSS), the FTS has added advantage of providing an early result with a view to Chorionic Villus Sampling so that an affected pregnancy can be diagnosed by the end of the first trimester.

What happens during the FTS test & how are the results interpreted?

A scan is first performed to measure the size of the baby and the Nuchal Translucency (NT). The length of the baby must be between 45mm to 84mm for a risk estimation to be valid. If the length is less than 45mm, a repeat scan would be arranged. If the length is more than 84mm, an alternative method of Down Syndrome screening (MSS after 16 weeks) would be discussed.

After the NT has been measured, blood will be taken from the mother to measure 2 hormones - PAPPA & beta HCG. An computer software accreditated by the Fetal Medicine Foundation in the United Kingdrom (http://www.fetalmedicine.com/f-fmf.htm) is used to calculate the overall risk of Down Syndrome for that pregnancy based on the maternal age, NT measurement and hormone levels. The final risk estimate is available after 2 working days.

Chorionic Villous SamplingThe final result will be given to the patient in the form of a risk. An arbitrary cut-off for this test is 1 in 250. A risk of 1 in 200 is a high risk result while a risk of 1 in 300 is a low risk result. A high risk result does not always mean that the baby has Down Syndrome. Mothers with high risk results have the option of an invasive test to confirm the diagnosis and it's the couple's choice if they wish to proceed.

These invasive tests include chorionic villus sampling (CVS) which is placental biopsy at 12 weeks or amniocentesis which is aspiration of the fluid around the baby at 16 weeks. These tests carry an estimated 1% risk of miscarriage.  

Is it compulsory to screen for Down Syndrome and what about older mothers?

The National Institute of Clinical Excellence (NICE) in the United Kingdom has emphasised the need to have in place by 2007 the capability of offering screening to all women in the first trimester using a combination of maternal age with the ultrasound marker nuchal translucency thickness (NT) and the maternal serum biochemical markers free beta-hCG and pregnancy-associated plasma protein-A (PAPP-A).

AmniocentesisPregnant women must now be given information about any screening or diagnostic tests that they are offered. The patient can decide not to have any tests at all, to have one test rather than another, or to have a screening test, but not a diagnostic test.

The American College of 0 &G (ACOG) states that "First Trimester NT/ freeBeta/PAPP-A screening should be offered to all pregnant women who are at low risk for the disorders being screened" : Therefore, the screening programs are for women who are under 35 years of age at the time of delivery and who do not have a positive history of chromosomal abnormality. Women who are over 35 years or have a positive medical history can consider amniocentesis for prenatal chromosome testing.

Screening is not a substitute for amniocentesis, because:
1) It does not give a diagnosis,
2) It estimates a risk for only 3 chromosomal abnormalities,
3) It will miss many of the hundreds of other types of chromosomal abnormalities,
4) It may falsely reassure some patients who still have an increased chromosomal abnormality risk due to age.
If the patient opts for an amniocentesis, the screening test is not necessary.

It needs to be recognised that some high-risk women will elect to have the screening tests instead of invasive diagnostic tests for various reasons, such as finding the risk of miscarriage associated with these tests unacceptable. Women above 35 who decline a diagnostic test can opt for the screening test as the result may assist in helping them decide whether to proceed with the diagnostic test.

Conclusion

While not all women will opt to have a screening test for Down Syndrome for various reasons, they should all be offered the test and be counselled of the aim and nature of the test. While the First Trimester Test (FTS) does not have a 100% detection rate, it is the test with the highest detection rate (90%) among the tests available in Singapore and all pregnancy patients should be educated about the availability of this screening test.

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