Down syndrome screening: understanding the difference between a risk result and a diagnosis

What is Down syndrome?

Down syndrome, also called trisomy 21, occurs when every cell in the body carries an extra copy of chromosome 21. Instead of the usual two copies, there are three. This additional genetic material affects how the brain and body develop, producing characteristic physical features and varying degrees of intellectual disability.

In over 95 percent of cases, Down syndrome arises from a random error in cell division when the egg or sperm is forming. It is not inherited in the conventional sense and is not caused by anything the parents did or did not do. The error redistributes chromosomes incorrectly, and it cannot be prevented.

People with Down syndrome have a wide range of abilities. With appropriate support, early intervention and good education, many learn to read and write, form relationships, hold employment and live semi-independently. Prenatal screening, when used, opens the possibility of preparing that support structure rather than only enabling decisions about the pregnancy itself.

How maternal age affects the risk

Maternal age is the strongest known risk factor for Down syndrome. As women age, the risk of a chromosomal non-disjunction error rises. Research published by Snijders and colleagues in Ultrasound in Obstetrics and Gynecology calculated trimester-specific risks by maternal age, providing the benchmark still used in clinical practice.

At age 25 the background risk is approximately 1 in 1200. At 35 it rises to roughly 1 in 350, and at 40 to approximately 1 in 100. These are population probabilities, not individual certainties. The great majority of women at every age deliver chromosomally normal babies.

Importantly, most children with Down syndrome are born to mothers under 35, simply because that age group accounts for the majority of all pregnancies. Age raises individual risk, but younger mothers contribute the larger share of cases by volume. Paternal age has a much smaller influence.

In the Middle East, high rates of consanguineous marriage contribute to increased prevalence of recessively inherited conditions, but consanguinity does not directly increase the risk of Down syndrome, which arises from a random chromosomal error rather than inherited gene variants.

Screening versus diagnosis: the essential distinction

This is the most important concept to understand before anything else. Screening tests tell you the probability that a condition is present. They do not confirm it. A result described as high risk or screen positive means the statistical likelihood has crossed a threshold, not that the diagnosis is made. A result of 1 in 50 means a 2 percent chance of Down syndrome and a 98 percent chance the baby does not have it.

A diagnostic test examines the baby's own chromosomes directly and gives a definitive yes or no answer. To obtain those cells requires an invasive procedure, which carries a small but real risk to the pregnancy. That trade-off is at the centre of the decision-making process.

Many parents receive a high-risk screening result and assume the diagnosis is confirmed. This misunderstanding causes enormous distress and sometimes leads to irreversible decisions made on incomplete information. Understanding from the outset that screening and diagnosis are categorically different protects against this.

First-trimester combined screening

The most widely used screening test in the first trimester combines a blood draw from the mother (measuring two proteins: PAPP-A and free beta-hCG) with an ultrasound measurement of the fluid at the back of the baby's neck, called the nuchal translucency (NT). These are fed into a computer algorithm alongside maternal age to produce a risk figure.

This combined test detects approximately 85 to 90 percent of Down syndrome cases with a false positive rate of around 3 to 5 percent. The false positive rate means that between 3 and 5 women out of every 100 screened will receive a high-risk result even though their baby does not have Down syndrome. They will then need to decide whether to proceed to a diagnostic test.

Nuchal translucency measurement requires specific training and calibrated equipment to be accurate. In settings where the operator has limited experience or the equipment is not properly maintained, the measurement can be unreliable and the risk calculation based on it misleading. If you have any doubt about the quality of the scan, requesting a referral to a specialist fetal medicine centre is appropriate.

A fetal anatomy scan done at around 18 to 20 weeks is not a trisomy 21 screening test in itself, but it can detect structural features associated with Down syndrome such as certain heart defects, a thick nuchal fold, or shortened long bones. These findings increase suspicion and may prompt discussion of further testing.

NIPT: cell-free fetal DNA testing

Non-invasive prenatal testing (NIPT) analyses small fragments of the baby's DNA that circulate in the mother's blood from around 10 weeks of pregnancy. By counting the relative amounts of chromosome 21 DNA, it can detect trisomy 21 with a sensitivity exceeding 99 percent and a false positive rate below 0.1 percent. A 2024 systematic review in PloS one confirmed that incorporating NIPT into antenatal screening programmes substantially improves detection rates.

Despite this high accuracy, NIPT is still a screening test, not a diagnosis. A positive NIPT result has not examined the baby's chromosomes directly. It must be confirmed by an invasive diagnostic test before any irreversible decision is made. Studies have documented that a measurable proportion of positive NIPT results are false positives, particularly for rarer chromosomal conditions.

The test requires a minimum proportion of fetal DNA in the blood sample, called the fetal fraction. This can be low in early pregnancy or in women with higher body weight, leading to inconclusive results that require a repeat draw. When fetal fraction is insufficient for analysis, the result is simply reported as no result rather than negative.

In the Middle East, NIPT is available in hospital laboratories in the UAE, Saudi Arabia, Kuwait and at select centres in Egypt and Jordan. In other countries the sample may need to be sent abroad, adding time and cost. When local access is unavailable, asking for a referral through a specialist obstetrician is the most reliable path.

Diagnostic confirmation: amniocentesis and CVS

If screening suggests a high risk, or if a family wants a definitive answer regardless of screening result, the options are amniocentesis and chorionic villus sampling (CVS). Both examine the baby's actual chromosomes and give a clear answer.

Amniocentesis is performed from week 15 onwards. The doctor uses ultrasound to guide a fine needle through the abdominal wall into the amniotic sac and withdraws a small amount of fluid. Fetal cells floating in that fluid are cultured and their chromosomes counted. The result takes one to two weeks for full karyotyping, though rapid FISH testing can give a preliminary result in 24 to 48 hours.

CVS is done earlier, between weeks 10 and 13, and takes a small sample of placental tissue through either the cervix or the abdomen. Because the placenta and baby share the same genetic origin, its chromosomes reflect the baby's. A systematic review by Salomon and colleagues published in 2019 found the procedure-related miscarriage risk to be approximately 0.5 percent for amniocentesis and 0.7 percent for CVS. These are real numbers that belong in any informed discussion.

A negative diagnostic result is definitive. A positive result confirms trisomy 21. In very rare cases the laboratory may find an ambiguous result, such as confined placental mosaicism, that requires additional specialist interpretation. The couple should be counselled before the procedure that such findings, though uncommon, are possible.

Women who choose not to undergo diagnostic testing because the risk to the pregnancy is not acceptable to them have every right to that decision. Equally, women who want certainty and are willing to accept the procedural risk have every right to proceed. The role of the clinical team is to provide accurate information, not to steer either way.

What a high-risk result actually means

When a screening result describes your risk as high or screen positive, the most important thing to understand is the actual number. A result of 1 in 50 means there is a 2 percent probability that the baby has Down syndrome and a 98 percent probability that it does not. Both parts of that statement are equally real.

Some families choose to proceed immediately to diagnostic testing. Others choose to wait and monitor, or to proceed with the pregnancy without further testing and prepare regardless of the outcome. All of these are legitimate responses, and a competent counsellor will present all options without preference.

The threshold that defines high risk is not biologically determined. Different screening programmes use different cut-offs: 1 in 250, 1 in 300, or 1 in 150. A woman just above the cut-off and a woman just below it have almost identical risk profiles, but one receives a high-risk letter and the other does not. Understanding that the threshold is a clinical decision boundary, not a medical verdict, is part of interpreting the result correctly.

After a positive NIPT result, the temptation to act immediately is understandable. But the evidence clearly shows that a proportion of positive NIPT results, particularly for conditions other than trisomy 21, do not confirm on diagnostic testing. Waiting for diagnostic confirmation before making any irreversible decision is the guidance all major obstetric organisations agree on.

Genetic counselling: why it matters

Genetic counselling is a consultation with a trained professional who explains what chromosomal test results mean in practical and statistical terms. It is not reserved for people who have received bad news. It helps any family with a high-risk screening result make an informed choice about the next step.

A genetic counsellor explains the actual probability behind a number, the implications of each test option including its procedural risks, what a confirmed diagnosis of Down syndrome would mean for the child's development and the family's life, and what resources and support services exist locally. The counsellor presents information without pushing a particular outcome.

In the Middle East, genetic counselling is available at teaching hospitals in Saudi Arabia, UAE, Jordan, Lebanon and Egypt. In countries with fewer specialist services, video consultations with centres in Beirut, Amman or abroad are an increasingly practical option. A referral from the treating obstetrician is usually the starting point.

Sources

• Sebire E, Rodrigo CH, Bhattacharya S et al: The implementation and impact of non-invasive prenatal testing (NIPT) for Down's syndrome into antenatal screening programmes: A systematic review and meta-analysis, PloS one, 2024
• Jayashankar SS, Nasaruddin ML, Hassan MF et al: Non-Invasive Prenatal Testing (NIPT): Reliability, Challenges, and Future Directions, Diagnostics (Basel, Switzerland), 2023
• Salomon LJ, Sotiriadis A, Wulff CB et al: Risk of miscarriage following amniocentesis or chorionic villus sampling: systematic review of literature and updated meta-analysis, Ultrasound in obstetrics and gynecology, 2019
• Coppede F: Risk factors for Down syndrome, Archives of toxicology, 2016
• Corder JP, Al Ahbabi FJS, Al Dhaheri HS et al: Demographics and co-occurring conditions in a clinic-based cohort with Down syndrome in the United Arab Emirates, American journal of medical genetics, 2017
• Snijders RJ, Sundberg K, Holzgreve W et al: Maternal age- and gestation-specific risk for trisomy 21, Ultrasound in obstetrics and gynecology, 1999
• World Health Organization: Congenital anomalies fact sheet