Down syndrome is the consequence of an atypical cell division. It is a common genetic condition first described by the English physician John Langdon Down. A set of specific symptoms that always occur together in individuals with this chromosomal anomaly has led it to be called “syndrome” or Down syndrome. This article highlights the screening and diagnostic tests used to detect Down syndrome during pregnancy.
What is Down Syndrome?
The human body functions like clockwork. Egg and sperm cells merge to form a zygote. Normally, a zygote contains 46 chromosomes, 23 from each parent, marking the beginning of life. However, sometimes, an arbitrary genetic glitch at the beginning of cell division causes an extra chromosome 21 to come into existence (trisomy 21). In these cases, the zygote carries 47 chromosomes instead of 46 (three copies of chromosome 21). Therefore, every cell in the baby’s body has an extra chromosome.
Answers to anxious questions like how to know if a baby has Down syndrome are that babies born with Down syndrome usually have low muscle tone (hypotonia) and bone development issues. They are born with a small head, a round, flat face, a flat nose, almond-shaped eyes, small ears, and a short neck. Their small mouth cavity makes the tongue appear larger, often protruding. These facial features are usually noticeable at birth.
These children are often shorter in stature, are prone to congenital heart defects, have difficulties with hearing and vision, thyroid issues, and respiratory infections. They struggle with mild-to-moderate intellectual disability, face developmental delays, and learn speech and motor skills at a much slower pace.
Types of Down Syndrome
Down syndrome has three variants. The most common type is Trisomy 21, where every cell in the individual’s body carries an extra strand of replicated chromosome 21. A total of 95% of all cases of Down syndrome belong to this category.
Translocation is a second and rarer variant of Down syndrome. In this case, part of chromosome 21 breaks off but does not exist as an individual third chromosome. It remains attached to another chromosome (often 14 or 21). This type of Down syndrome affects only about 3 to 4% of all affected individuals. Translocation Down syndrome is usually inherited from an unaffected parent carrier.
There are few and far between cases of Mosaic Down Syndrome, only about 1 to 2%. In Mosaicism, only a handful of cells have the extra chromosome. This results in milder symptoms.
Ways to Detect Down Syndrome
The prenatal screening for Down syndrome indicates an increased risk. Diagnostic tests confirm the presence.
- Noninvasive Prenatal Testing (NIPT) is done at 10+ weeks of pregnancy. This blood test analyses the fetal DNA in the mother’s blood.
- The Down syndrome test during pregnancy involves blood and ultrasound tests during the first trimester or between the 11th and 14th week of pregnancy. A blood test is done to check for certain proteins in the mother’s blood that suggest a higher chance of Down syndrome.
- The ultrasound test utilises high-frequency sound waves to detect the fluid at the back of a fetus’s neck, which might indicate Down syndrome.
- Another combined test is done when the gestation age is 15 to 20 weeks. This test during the second trimester is known as the Quad screen or integrated test. This blood test measures four factors to assess risk. Doctors use both the blood and ultrasound test reports from the first trimester and the second-trimester blood test. They compare and assess these reports to produce one Down syndrome risk rating.
In case a woman is pregnant with twins or triplets, the blood test during the second trimester will not be considered reliable, because the substances from a Down syndrome fetus may be harder to detect in such a scenario.
If screening tests, NIPT, First Trimester Screening, and Quad suggest the likelihood of Down syndrome, a diagnostic test is performed. Diagnostic tests often carry a low risk of miscarriage. Hence, the expectant couple might choose to discuss their family history with a genetic counsellor to get a clear idea about the risks and benefits of the tests.
- Chorionic Villus Sampling (CVS), a diagnostic test done during the 10th to 14th week of pregnancy, answers the question of how Down syndrome is detected during pregnancy. This test involves removing a sample of placental cells. The sample is tested for extra material from chromosome 21, which may indicate that the fetus has Down syndrome.
- Amniocentesis is the diagnostic test done during the second trimester at 15+ weeks. A sample of the amniotic fluid is extracted and analysed. This test confirms whether the baby has Down syndrome.
- Percutaneous Umbilical Blood Sampling or PUBS is done within the 18th to 20th week of pregnancy. A sample of foetal blood in the umbilical cord is extracted through the mother’s uterus to ascertain whether the extra chromosome exists. Although risky, PUBS can confirm the results of CVS and Amniocentesis.
Conclusion
Down syndrome does not hinder one from leading a happy and accomplished life. Speech and occupational therapy, physical training, and personalised medication address developmental issues and health conditions. Educational programs ensure cognitive development. Behavioural therapy supports social and emotional growth.