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Aneuploidy Screening

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Aneuploidy screening

Paras Advanced Centre for Fetal Medicine offers screening tests and diagnostic tests for Down Syndrome.

A screening test estimates the risk of your baby having Down Syndrome. It only tells you whether or not you are at particularly high risk of having a baby with Down Syndrome. This does not confirm the chromosome status of your baby.

A diagnostic test can give you a definite diagnosis, but also carries a small risk of miscarriage.

Screening Tests

There are many different screening tests available. There are ultrasound tests, blood tests and a combination of the two.

The ultrasound tests include the nuchal translucency or the first trimester screening scan between 11- 14 weeks of pregnancy and the anomaly scan or second trimester screening between 18-20 weeks. In these scans, apart from the general growth and development, we look for markers for Down Syndrome in the fetus and depending on the presence or absence of these markers, we assess the risk for Down Syndrome.

The blood tests include the first trimester test which measures two hormones in the mother – beta HCG and PAPP-A between 10-14 weeks of pregnancy or the second trimester triple serum screening in which three hormones (beta HCG, AFP and uE3) are measured in the maternal blood between 15 – 20 weeks.

If both ultrasound and blood tests are done, then the risk assessment obtained from both can be combined to get a more accurate final risk assessment.

Diagnostic Tests

The diagnostic test is either a CVS (Chorionic Villus Sampling) or Amniocentesis which will confirm the chromosomal status of the fetus and tell you definitely whether or not your baby has Down Syndrome.

However, these are invasive tests and they carry a risk of miscarriage. Hence they are generally offered only if the screening tests are suggestive of a problem.

What is Down’s Syndrome

Down Syndrome (DS) is a chromosomal disorder in which the affected individual has one extra chromosome in addition to what is the normally allocated number of chromosomes. Every human being has 46 chromosomes arranged in 23 pairs. In people with Down Syndrome, the chromosome pair number 21 has three copies instead of two so that the total number of chromosomes is 47 instead of the normal 46. Each man’s sperm has 23 chromosomes and so does each woman’s egg, so that when conception occurs and the sperm fertilises the egg, a new human being with a full complement of chromosomes is formed. Sometimes mistakes occur and lead to what are described as chromosomal abnormalities. Down Syndrome is one of the commonest of these. At conception, instead of one number 21 chromosome from the father and one from the mother coming together, a third chromosome creeps in and is then duplicated in every cell of the baby’s body. Down Syndrome, also called Trisomy 21, is the most common chromosomal abnormality, occurring approximately once in every 600 births.

Why should we test for Down’s syndrome?

Down Syndrome is the most common chromosomal abnormality and can have a major social, financial and emotional impact on a family. It is the commonest cause of global developmental delay in children, which is NOT inherited.

What are the characteristics of Down’s syndrome?

People with Down Syndrome will all have some degree of learning difficulty. Many people with Down’s will go to ordinary schools and lead semi-independent lives, but others will need full-time care. Certain medical conditions are more likely to occur in people with Down Syndrome. These include:

  • heart defects, which occur in one in three children with Down’s
  • digestive tract defects
  • problems with sight
  • problems with hearing

All people with Down’s will have certain physical characteristics, such as eyes that slant upwards and outwards, a single crease running across the palm of the hand, low-set ears and small hands.

Babies born today with Down Syndrome can expect to live between 40 and 60 years on average.