The human body is composed of organs, each made up of tissues, which in turn consist of cells. Each cell operates as a tiny factory where all the processes that keep us functioning take place. All the instructions a cell needs to work and to form itself are found in the cell nucleus in the form of ‘recipes’- the genes – which are organized into 23 pairs of ‘encyclopaedias,’ known as chromosomes.
Down Syndrome, or trisomy 21, occurs, in most cases, due to an extra chromosome 21, resulting from an error in the formation of sex cells (sperm and eggs).
This extra chromosome leads to developmental delay, intellectual disability, characteristic physical features, and a higher risk of congenital malformations and other health issues in childhood and adulthood. It can be confirmed through a test called a karyotype (microscopic examination of chromosomes). This diagnostic test can be conducted after birth with a blood sample or during pregnancy through invasive procedures, such as amniocentesis or chorionic villus sampling (CVS).
Trisomy 21 is a genetic condition but is not inherited: the error is usually sporadic, and in most cases, there is no increased risk due to family history, nor is it rare: the incidence of the syndrome at birth is 1 in 700. The most significant risk factor is maternal age.
Thus, since the 1970s, amniocentesis has become a standard procedure for pregnant women over 35. Chorionic villus sampling emerged later. However, because these tests are invasive, they carry a risk of miscarriage and require specialized equipment and staff. In the 1990s, increased nuchal translucency (an ultrasound finding) was introduced as a risk marker, evolving into a more complex first-trimester screening that combines biochemical analysis and other ultrasound markers, integrated in sequential or contingent screening. Despite a substantial reduction in invasive testing, 5% of high-risk pregnancies still undergo it due to false positives, and an important residual risk remains (sensitivity of these screenings does not exceed 90%).
In 1995, cell-free DNA (cfDNA) in maternal blood was first described.
However, it was not until 2008, with the advent of next-generation sequencing, that a test we now know as NIPT became feasible. Today, non-invasive prenatal testing (NIPT) is the gold-standard test with the highest specificity and sensitivity for trisomy 21 (>99.9%) and is recommended for all pregnant women, regardless of age or risk.
So, what is NIPT?
All fetal DNA (deoxyribonucleic acid) is represented in small fragments that result from the death of trophoblast cells (placenta). This cell-free fetal DNA can be detected and studied from the 9th week of gestation (up to the end of pregnancy, without a specific time window like earlier screenings) and is rapidly cleared after birth, allowing its use for each pregnancy. The test is conducted on a maternal blood sample (non-invasive), posing no risk to the fetus, and can also be used in twin pregnancies or those resulting from assisted reproductive technologies.
It is now standard practice in many countries (over fifty), though implementation varies primarily due to economic factors. There are also concerns regarding health literacy, pre-test genetic counselling, and the resources required.
It is crucial for couples to be informed about the test’s capabilities, limitations, and potential unexpected incidental findings. Examples include mosaicism (where detected changes do not necessarily reflect the fetus but are confined to the placenta), vanishing twin syndrome (where cfDNA from a twin that passed in utero may be detectable up to 16 weeks), maternal malignancy (anomalous NIPT results may suggest maternal cancer), and absence of a result (the inability to obtain results is a risk factor). It is also important to remember that cfDNA originates from the placenta, making NIPT a screening test (despite its high sensitivity of 99.9%) that requires invasive confirmation of results. Moreover, excluding trisomy 21 does not eliminate other risks for fetal conditions (the risk of congenital malformations in any pregnancy is 2.5%, while trisomies account for only 0.3%) and maternal conditions that can affect both mother and fetus (such as maternal hypertension, gestational diabetes, or viral infections). Therefore, a negative result should not provide false reassurance or justify less vigilance.
This counselling will become increasingly challenging with the emerging applications of NIPT: screening for rare aneuploidies, deletions, microduplications, and monogenic diseases (from Rh factor determination to studying de novo or recessive dominant diseases).
NIPT is undoubtedly the most promising technology in prenatal diagnosis, significantly reducing the need for invasive tests and equipping both prospective parents and doctors with essential information for decision-making during pregnancy. Its growth is inevitable, so it is essential to address and find solutions to the associated challenges: literacy, implementation models, and costs.
On 1 March, Portugal’s Directorate-General of Health (DGS) published guidelines allowing all pregnant women in the national health system to access NIPT, contingent on an intermediate risk from first-trimester combined screening (between 1:101 and 1:1000). In my opinion, this policy is overdue and falls short, but it represents an important step in our health policy.
