Importance of prenatal screening to detect down syndrome

Sachin Chaurasiya, Genetic Counselor, Lilac Insights explains that over the years prenatal screening techniques have improved significantly and today they can detect cases of Down syndrome with a sensitivity of up to 99 per cent. The prenatal screening programme is intended to ensure that expecting parents are aware of their yet to be born child’s condition well in advance

Down syndrome, also known as Trisomy 21 is a genetic disorder that occurs due to the presence of an “extra chromosome”. Instead of 23 pairs, i.e., 46 chromosomes, the individual has an extra chromosome number 21, making a total of 47. The presence of an extra chromosome can cause many physical problems along with developmental and intellectual delays. People with Down syndrome also have a higher likelihood of developing health problems like heart disease, dementia, and issues with the thyroid.

In India, we have approximately 25 million births taking place every year. On average, there is one child born with Down syndrome for every 700 births, which means India could have around 35,000 babies with Down syndrome born per year.

Over the past 10 years, India has experienced a rapid change in the family dynamics, the number of nuclear families has increased and thus it has become challenging to raise a child with Down syndrome if both the parents are working or do not have the resources to seek care. Further, women are opting to plan a pregnancy at later stages of their life. Today with changing family dynamics and increasing maternal age at term, the importance of prenatal screening cannot be ignored.

Over the years prenatal screening techniques have improved significantly and today they can detect cases of Down syndrome with a sensitivity of up to 99 per cent. The prenatal screening programme is intended to ensure that expecting parents are aware of their yet to be born child’s condition well in advance. The programme enables the couple to decide on various options available for the pregnancy and also helps them in making informed decisions during the pregnancy.

Prenatal screening involves a blood test and foetal scan for best results. Screening tests generally mention whether there is an ‘increased/high risk’ or a ‘low risk’. A ‘high risk’ result indicates that the birthing individual needs to be monitored more closely or offered further confirmatory tests. A “Low risk” is meant to be reassuring with less intense monitoring. This is a method for triaging pregnancies so that specialised care can be offered to “high-risk pregnancies”. The definitive or confirmatory test for a chromosomal problem in the fetus is available only by an invasive test that can be considered at either 12 weeks or 16 weeks of gestation i.e., a Chorionic Villous sampling (CVS), which takes a small amount of tissue from the placenta or the Amniocentesis, which takes a small amount of amniotic fluid.

Laboratories or clinics that carry out prenatal screenings need to have specified permissions; therefore these screening and diagnostic procedures can only be carried out in registered genetic clinics and by qualified and registered persons under the pre-conception and prenatal diagnostics Techniques Act & Rules 1994 /1996 (PCPNDT Act/Rules).

With pre-natal screening, parents are better prepared to welcome a child with Down syndrome. Parents with positive or ‘high risk’ results are also strongly advised to talk to their doctor or a genetic counsellor to understand their options and if there is a need for further testing.

The prenatal screening tests available in India include;

1. Biochemical based screening test- These tests evaluate the serum values of various hormones at different stages of the pregnancy to calculate the pregnancy-specific risk or common aneuploidies. Biochemical based screening tests can further be differentiated based on the period at which they can be performed.

• First Trimester Screening (FTS)- Performed between 10-13⁺⁶ weeks of gestation, the test included are Double Marker, First Trimester quad and the Preeclampsia screening.

• Second Trimester Screening (STS)- Performed between 15+21⁺⁶ weeks of gestation, the test included are Triple and Quadruple Marker.

2. NIPS (Non-Invasive Screening test)-The test can generally be performed anytime after 10 weeks of gestation although an ideal time suggested is after an NT scan (i.e 12-week scan).

It should be noted that each test has its pros & cons therefore it is suggested to have these options discussed with your health care provider or a genetic counsellor.