Non-Invasive Pre-Natal Testing For Down Syndrome

What is Down Syndrome?

Down syndrome is the most common genetic abnormality in human beings. It is a disease in which an extra copy of chromosome 21 is present, making the total chromosome count change from the normal 46 to abnormal 47. Also known as Trisomy 21, it affects around 1 in every 800-1000 live births in the US.

Features:

DS has some characteristic features that make diagnosis easy. These include:

a. Physical- The babies affected by DS have a flat head, slanted eyes, flat nose and a large protruding tongue. Hands have a characteristic single palmar crease. Muscles are weak toned. Overall the stature is short, and growth is stunted.

b. Neurological- Almost all DS patients are mentally impaired and have an average IQ of only 50. They are more susceptible to epileptic seizures and degenerative brain diseases like Alzheimer’s.

c. Heart- About 40% of DS babies have structural heart deformities. The most common of which are atrioventricular septal defects. Less commonly, valvular defects are also found.

d. Reproductive- Males with DS usually have undescended testes and are infertile. Females are fertile but can rarely conceive.

What is NIPT?

Non-Invasive Pre-Natal Testing for Down’s (NIPT) is a 99.6% sensitive and 99.9% specific test carried out in pregnant females to check for genetic abnormalities in the fetus. It is not a diagnostic test, but rather a screening test that detects fragments of fetal DNA amid maternal DNA. It can be performed after the 10th gestational week.

How does it work?

Fetal DNA is about 10% of total DNA circulating in maternal blood. This DNA, being different from that of the mother, is easily identified. In NIPT, blood is drawn from the mother and the amount of the fetal DNA present is calibrated, checking if any chromosome or part of a chromosome is either missing or present in excess. This method detects any quantifiable genetic abnormality present, most commonly being a trisomy 21, 18 or 13, or missing or extra sex chromosome.

Role of NIPT:

NIPT is highly accurate when it comes to screening for DS. It is a non-invasive procedure because it is a simple blood test and does not come in direct contact with the growing fetus. If the number of DNA fragments of any chromosome is more than 3 standard deviations from the mean value, when matched with reference chromosomes, the test is considered to be positive.

If the NIPT results come back positive, then invasive diagnostic tests like Amniocentesis and Chorionic Villus Sampling are recommended. If the test comes negative, then such invasive tests are not required.

Who Should Have NIPT Carried Out?

NIPT is an extremely useful and accurate but a very expensive screening test. So this test is recommended for expectant mothers who are at a high risk of having fetal abnormalities. High risk features include:

– Maternal age- The more aged the mother, the more likely her child will have DS.

– A child with DS- If a couple already has a child with DS, it is likely that subsequent pregnancies will have genetic abnormalities as well.

– Carrier Parent- Males with DS are usually infertile but females are not. Although females with DS rarely conceive, but if they do, they are likely to transfer the defective gene to their offspring.

Conclusion:

Down syndrome is a disease that greatly reduces the quality of life of both the parents and the affected child. Simple measures like prenatal screening of DNA can detect this genetic disease and make the parents better prepared to deal with the situation.