Risk vs. certainty: Why understanding prenatal screening and diagnosis matters

Advances in prenatal medicine have significantly transformed how chromosomal conditions such as Down syndrome are identified during pregnancy, allowing expectant parents to access earlier and increasingly accurate assessments than ever before.

Globally, Down syndrome remains one of the most common chromosomal conditions, occurring when a baby is born with an extra copy of chromosome 21, a genetic change that can affect physical growth, learning ability, and overall health.

With the growing availability of sophisticated prenatal testing technologies in India, doctors say one of the biggest areas of confusion among families is understanding the difference between screening tests and diagnostic tests for Down syndrome.

According to specialists, screening tests estimate the likelihood or risk that a foetus may have the condition, whereas diagnostic tests are designed to confirm whether the baby is actually affected.

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“Prenatal screening helps identify pregnancies that may have a higher chance of chromosomal abnormalities, but screening itself is not diagnostic,” said Dr. Talat Khan, Medical Genetics, Metropolis Healthcare Limited.

One of the earliest and most widely discussed screening methods today is Non-Invasive Prenatal Testing (NIPT), also known as Non-Invasive Prenatal Screening (NIPS) which can be performed after 10 weeks of pregnancy.

“NIPT analyses cell-free fetal DNA circulating in the mother’s blood and is highly sensitive, especially in high-risk pregnancies,” said Dr. Khan.

The test has gained popularity globally because it only requires a maternal blood sample and poses no direct risk to the pregnancy. Experts say it has significantly improved prenatal screening accuracy for Down syndrome compared with older biochemical screening methods.

Another commonly recommended early screening option is the First Trimester Screening or Dual Marker Test, typically performed between 11 and 13.6 weeks of pregnancy. This combines maternal blood testing for biochemical markers such as free beta hCG and PAPP-A along with the Nuchal Translucency (NT) ultrasound scan.

In the second trimester, doctors may also advise the Quadruple Marker Test, usually conducted between 15 and 20 weeks. This evaluates four biochemical markers such as AFP, hCG, unconjugated estriol (uE3), and Inhibin A  to estimate the risk of chromosomal abnormalities.

Dr. Sunita Bijarnia-Mahay, Senior Consultant, Vice Chairperson and Professor, Clinical & Metabolic Genetics at Sir Ganga Ram Hospital, said prenatal screening is important for all pregnant women regardless of maternal age.

“Prevention and early preparedness are extremely important for a child’s health. Prenatal screening is therefore advised for all pregnant women, whether young or elderly,” she said.

However, she added that traditional maternal serum screening tests such as Dual, Triple or Quadruple Marker Tests have relatively lower sensitivity levels ranging between 60 and 80 percent.

“Ultrasound screening during the 11–13 week window can also help identify certain markers but may still miss some cases,” she explains.

“In comparison, NIPS or Non-Invasive Prenatal Screening has significantly improved screening accuracy. It checks fetal DNA from the mother’s blood and is nearly 99 percent accurate in detecting Down syndrome.”

The test is now increasingly available across major fetal medicine and genetic centres in India operating under PCPNDT regulations.

Despite their high accuracy, experts caution that screening tests — including NIPT — are still not confirmatory.

A high-risk screening result does not necessarily mean the baby has Down syndrome, and similarly, a low-risk result cannot completely eliminate the possibility.

This is where diagnostic testing becomes crucial.

Dr. Talat Khan said that procedures such as Chorionic Villus Sampling (CVS) and amniocentesis are considered diagnostic tests because they directly analyse fetal samples collected during pregnancy.

“These tests use advanced laboratory techniques such as QF-PCR, FISH, karyotyping, and chromosomal microarray analysis to confirm chromosomal abnormalities,” she says.

Unlike screening tests, diagnostic procedures can provide definitive confirmation. However, because they are invasive, they may carry a small procedure-related risk and are usually recommended when screening results indicate elevated risk or when clinically indicated.

“All screening tests, if found positive, must be followed by confirmation through invasive diagnostic testing such as amniocentesis,” said Dr. Sunita Bijarnia-Mahay.

Doctors also emphasised the importance of genetic counselling before and after prenatal testing.

Experts say counselling plays a critical role in helping families understand the purpose, benefits, limitations and implications of each test, while also reducing unnecessary anxiety and confusion.

“A positive screening result should always be viewed as an indicator for further evaluation rather than a final diagnosis,” Dr. Talat Khan added.

As prenatal testing technologies continue evolving, specialists believe awareness about the distinction between screening and diagnosis is becoming increasingly important in reproductive healthcare.

Understanding what each test can and cannot determine may help families navigate pregnancy decisions more responsibly, avoid panic over preliminary results, and ensure timely medical guidance wherever necessary.