Non-invasive screening for Down syndrome
Down syndrome is a congenital condition where a child is born with an extra chromosome, which is associated with developmental and intellectual delays. In this article, a consultant in fetal medicine and obstetrics, Dr Elena Greco, explains how Down syndrome can be tested for before birth with non-invasive methods.
What is Down syndrome and what causes it?4
Down syndrome is a lifelong condition with no ‘cure’ and it varies in severity among individuals. Some individuals with Down may require round-the-clock caretaking, and others may be functionally independent.
The extra chromosome that causes Down syndrome is a third, additional copy of chromosome 21. Typically, humans have 23 pairs of chromosomes – one half from the sperm, and one half from the egg. Unusual cell division leads to an additional chromosome, which is called trisomy 21 and causes genetic changes which impact the development of the body and brain. Parents who are older in age have a higher chance of conceiving a child with Down syndrome due to the deterioration of egg and sperm quality over time. In the majority of cases, it is not an inherited condition, and it can happen to any baby during fetal development, occurring randomly. It is not expected that individuals with Down syndrome will pass down their condition to their biological children. There is an uncommon type of Down syndrome, called translocation Down syndrome, where parts of chromosome 21 become attached (‘translocated’) to another chromosome before or at conception, and with which it is possible that the condition can be inherited by the children.
What are the symptoms of Down syndrome?
Symptoms of Down syndrome consist of cognitive, developmental, and physical aspects.
Children with Down syndrome will take longer to reach milestones such as talking, sitting upright, and walking. They may likely have issues with memory retention, learning new things, focus, and problem-solving.
Physical attributes that are associated with, but not exclusive to, Down syndrome include:
- A flattened face
- Small nose with a flat bridge
- Small head and short neck
- Almond-shaped eyes that are upward slanting
- Palms with a singular crease across (called the simian crease) as opposed to the ‘heart’ and ‘head’ lines
- Short in stature
- Loose and over-flexible joints
Down syndrome, if not caught prenatally, is almost always diagnosed in infancy. It is very rare to be diagnosed as an adult, but it is possible if the symptoms are mild.
Down syndrome can also be linked to a variety of medical issues of the heart, immune system, and digestive system, due to its effect on the body’s structure. These can include heart structural issues that cause regurgitation, gastroesophageal reflux disease, autoimmune disorders, sleep apnoea, cataracts, early-onset dementia, and a spinal condition called atlantoaxial instability.
Can Down syndrome be screened for before birth?
Expecting parents can screen for the risk of their babies having Down syndrome with prenatal testing. Prenatal testing can start from 10 weeks of pregnancy at the earliest.
Prenatal testing can be performed in a way that does no harm to the mother or the fetus, which is why it is often referred to as ‘non-invasive prenatal testing’ (NIPT). It involves using a sample of blood from the pregnant mother, with which doctors can check for congenital abnormalities within the fetus’s DNA – not just Down syndrome, but Edward’s syndrome and Patau’s syndrome as well. The results can be determined with the mother’s blood because, during pregnancy, it will contain DNA from the placenta, where the fetus is gestating.
The NIPT can be combined with a scheduled ultrasound scan (such as the 12-week scan). It can be helpful to have the ultrasound scan as it can further inform the results of the blood test. The ultrasound will measure the fluid at the back of the fetus’s neck to get the nuchal translucency – a lot of fluid build up is also associated with chromosomal or genetic congenital conditions. However, this measurement may not always be taken as it depends on the fetus’s positioning. The blood sample is taken from the mother’s arm, meaning that no needles come near the fetus.
What do the results of non-invasive prenatal testing mean?
There are two possible results from the test: low chance, where the chromosomal reading does not indicate a likelihood of the child having Down syndrome, and high chance, where the chromosomal reading of the blood has found more DNA that would indicate one pair of chromosome 21, and thus there is a higher chance that the child will have Down syndrome but it is not a guarantee. However, NIPT has been proven to be quite accurate, with 90 per cent of children that were tested as having a high chance being born with Down syndrome.