Antenatal Screening Tests for Chromosomal Abnormalities

By Dr. Loke Kah Leong
MBBS (S'pore), MMed (O&G), MRCOG (UK), FAMS (S'pore), FICS (USA), FRCOG (UK)

There are a few screening tests for chromosomal abnormalities available which are offered to all pregnant women in the late first trimester or early second trimester. These include the nuchal translucency (NT) scan, the NT combined with maternal blood test commonly known as the OSCAR test or maternal serum screening along (eg. Triple Test).

Screening tests are all non-invasive and there is no risk of causing miscarriage to the pregnancy. As the risk of chromosomal abnormalities rises with maternal age, screening tests assume an even greater importance nowadays as more women are having babies at a later age.

Nuchal Translucency (NT)

The NT test involves a detailed ultrasound scan of the baby at 11- 14 weeks of the gestation and the fluid collection behind the baby’s neck is measured. A thickened NT beyond normal thickness is associated with chromosomal abnormalities, Down’s syndrome being the most common, although Edwards’s syndrome Patau’s syndromes are also screened. The normal thickens is usually less than 2.5mm.

A risk estimate can be obtained by the result of the fetal NT value, maternal age and the crown-rump length of the fetus. The accuracy of the NT test alone is about 80%.

OSCAR

The OSCAR test involves the NT test plus a sample of the mother’s blood which is analysed for levels of free beta hCG and PAPP-A (Pregnancy associated plasma protein A). The combined data will give a risk estimate of Down syndrome with an accuracy of 90% which is superior to that NT alone (80%)

Maternal Serum Screening

The maternal serum screening test is done between 15-20 weeks of gestation and this measure certain hormones in the blood to determine risk. The triple test measures three hormones called alpha-fetoprotein, human chorionic gonadotrophin and oestriol and a risk value is calculated according to maternal age and the gestational age of the fetus at that time.

As its accuracy is only 65%, this test is less commonly done nowadays. Furthermore, it has to be done later from 15 weeks onwards compared to NT and OSCAR which can be done from 11 weeks onwards.

Interpretation of the Results

The result of the screening test will give you a numerical risk value which tells you if your fetus belongs to a low risk or a high risk group. Screening tests are NOT diagnostic tests and cannot tell you if your baby has Down’s syndrome.

A result of 1 in 300 means that one of 300 women will have an affected baby. Similarly if also means that 299 out of 300 women with this results will NOT have an affected baby.

From a medical standpoint, the cut-off point between high and low risk is taken at 1:300. Therefore, 1:50 would be considered high risk and 1:1000 would be low risk. It is important to understand clearly that the results of screening tests represent risks.

A low risk does not absolutely exclude the possibility of Down’s syndrome or other abnormalities altogether. Conversely, a high risk result does not mean the fetus is affected and further diagnostic invasive tests (Eg Chorionic villus sampling or amniocentrsis) must be done.

Invasive tests obtain cell samples from the fetus and the results will be able to confirm if the baby is affected with Down’s syndrome. The risk of miscarriage related to the procedure is 0.5%. A normal result would certainly give the parents peace of mind while an abnormal result will allow the parents the option of whether to continue the pregnancy or have termination.

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