Learn about the need for Prenatal screening and its role in ensuring a safe pregnancy.


  • The initial 3 months of pregnancy is called the first trimester.
  • At this time a blood test and an NT scan is conducted between 11 weeks and 13 weeks of pregnancy.
  • The blood test provides a picture of the baby’s chromosomal structure called a karyotype.
  • The NT measurement, along with the patient’s age and dates of the pregnancy, are used to calculate the risk for chromosomal disorders.
  • The stage of pregnancy from 16 to 20 weeks is called the Second Trimester.
  • At this stage, a blood test called the triple/quadruple screening is performed along with an anomaly scan to rule out structural malformation in the fetus.
  • If the blood test results indicate a low risk of Down’s Syndrome, it does not guarantee that the baby will not be born with the condition

Pregnancy is a time for joy and celebration. The excitement also brings with it a mix of feelings including a sense of anxiety and caution to stay aware and safe. Expectant mothers and their families' requests for reassurance is perfectly understandable and it is for this purpose that Obstetricians and Fetal Medicine Consultants work together closely to ensure a safe pregnancy.

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An expectant mother undergoes many physical and hormonal changes. Obstetricians regularly perform a check-up or prescribe tests to gauge the development of the fetus. The Prenatal screening is one such test that is critical to understand the early development of the fetus.

What constitutes Prenatal screening for chromosomal abnormalities?

First trimester Screening (11 to 14 weeks)

The initial 3 months of pregnancy is called the first trimester and is one of the most important phases. At this stage a blood test and ultrasound conducted between 11 weeks and 1 day to 13 weeks 6 days of pregnancy provides an estimate of the chance that a pregnancy will have a chromosome condition, specifically, Down syndrome, Trisomy 18 or Trisomy 13.

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Down's syndrome (Trisomy 21), Edward syndrome (Trisomy 18) and Patau syndrome (Trisomy 13) occur when there is an extra chromosome resulting in birth defects and varying levels of intellectual disability. Babies with Down's syndrome have distinctive physical features with mild to moderate intellectual disability and may also have other birth defects such as heart problems.

How to identify Down's Syndrome

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In cases of babies with Down's Syndrome, there are three copies of chromosome 21. Such people have 47 chromosomes instead of the usual 46 (23 pairs). The extra chromosome appears with the 21st pair. 40% of Down's Syndrome cases do not exhibit structural abnormalities in scan.

Trisomy 18 and Trisomy 13 are less common than Down’s syndrome but they are serious conditions that include birth defects, severe disability, and shortened lifespan which cases most of the time can be diagnosed during the prenatal period itself.

Fetal Medicine specialists check the nuchal translucency (skin thickness at the back of the neck) and nasal bone of the fetus to assess the risk of the Fetus having Down’s syndrome.

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A measurement of the fluid underneath the skin along the back of the baby's neck will be taken. This measurement is called the nuchal translucency measurement. All babies will have some fluid here, but in some pregnancies with chromosome conditions or other health concerns, there tends to be more fluid and/or the absence of the nasal bone. This nuchal measurement, along with the patient's age and dates of the pregnancy, are used to calculate the risk for Down’s syndrome, trisomy 13 and trisomy 18.

If the result is one in 250 or higher, it is typically recommended to undergo an invasive diagnostic procedure such as Chorionic villus sampling (CVS) or Amniocentesis which carries a small percentage risk of miscarriage. In these procedures, guided by scan, a needle is inserted in the woman's uterus to draw out either amniotic fluid (amniocentesis) or tissue from the placenta (CVS). The fluids contain cells from the baby which are examined for the chromosomal abnormality.

prenatal diagnosis - CVS And Amniocentesis

The non-invasive prenatal testing is a blood test that involves drawing 20ml of the mother's blood to test the Fetal cells. This method is 99.99% sensitive and is specific only for Down's Syndrome testing.

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Second Trimester (16 To 20 Weeks) Screening

While the patients who have missed the tests during the first trimester can opt for it during the second trimester, the test sensitivity for Down's Syndrome is lower than in the first trimester.

In the second trimester, quadruple screening is performed after an early anomaly scan to rule out structural malformation in the fetus.

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A low risk of Down's Syndrome does not guarantee that the baby will not be born with the condition and likewise an increased risk does not guarantee that the baby will be born with the condition.

This is only a screening test and if required confirmatory test has to be performed after a detailed consultation with Fetal Medicine expert.

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