Prenatal Testing and Diagnosis
There are two types of tests for Down syndrome that can be performed before the birth of a child: screening tests and diagnostic tests.
Prenatal Screening Tests
Screening tests do not provide a confirmed diagnosis of Down syndrome — they indicate probabilities only.
There is a long list of screening tests available to pregnant women today, most of which involve a blood test and an ultrasound scan. Blood tests (or serum screening tests) measure the amounts of various substances in the mother's blood, including alpha-fetoprotein, inhibin A, pregnancy-associated plasma protein A (PAPP-A), estriol, and human chorionic gonadotropin (hCG). These results, combined with the mother's age, are used to calculate and estimate the probability of having a child with Down syndrome. These tests are typically performed during the first and second trimesters of pregnancy, but they can detect the possibility of Down syndrome in the fetus with approximately 80% accuracy. No prenatal screening test can definitively diagnose Down syndrome.
Blood tests are usually performed alongside a detailed ultrasound scan to look for any "markers" — characteristics that some researchers believe may be associated with Down syndrome. Researchers have recently developed a comprehensive maternal screening approach that combines serum blood testing, ultrasound, and the mother's age, significantly improving diagnostic accuracy in the early stages of pregnancy. Even so, this test is still unable to definitively diagnose Down syndrome.
Several companies specialise in cell-free DNA (cfDNA) testing, which is available only by medical referral and involves analysing a blood sample taken from the pregnant mother at around ten weeks of pregnancy. The test detects any fetal cell-free DNA circulating in the mother's bloodstream. According to the latest research, this test can detect 98.6% of cases of Trisomy 21. A "positive" result means there is a 98.6% probability that the fetus carries a third copy of chromosome 21, while a "negative" result means there is a 99.8% probability that it does not. If the result is positive, mothers can confirm the diagnosis through one of the diagnostic procedures described below.
Prenatal screening tests are now available to women of all ages. If a screening test indicates an elevated probability of having a child with Down syndrome, doctors will most likely recommend that the mother undergo a diagnostic test.
Diagnostic Procedures
Diagnostic procedures are carried out to confirm the results of prenatal screening tests. They include chorionic villus sampling (CVS) and amniocentesis (analysis of the amniotic fluid surrounding the fetus). These procedures are approximately 100% accurate in detecting Down syndrome, though they carry a 1% risk of spontaneous miscarriage. Amniocentesis is typically performed during the second trimester, between weeks 15 and 22 of pregnancy, while chorionic villus sampling is performed between weeks 9 and 14. Both tests can also distinguish between the genetic types of Down syndrome: Trisomy 21, Translocation Down syndrome, and Mosaic Down syndrome.
Diagnosing Down Syndrome After Birth
Because many parents choose not to undergo prenatal screening, the majority of Down syndrome cases are diagnosed after birth. Doctors typically suspect Down syndrome when certain physical characteristics are observed.
Some common characteristics shared by children with Down syndrome include:
- General low muscle tone (hypotonia)
- A flattened facial profile
- A small nose
- Upward-slanting eyes
- A single deep crease across the centre of the palm
- Increased joint flexibility
- Small skin folds at the inner corners of the eyes (epicanthal folds)
- A gap between the big toe and the adjacent toe
These characteristics alone are not a confirmed diagnosis of Down syndrome. Doctors must therefore carry out a specific test called a "karyotype" before confirming the diagnosis. This test involves taking a blood sample to examine the child's cells, using specialised tools to photograph the chromosomes and classify them by size, number, and shape. This analysis accurately determines whether the child has Down syndrome.
Source: The information in this article has been reproduced under an exclusive agreement with the National Down Syndrome Society. Available at: ndss.org
