How Accurate Is the Combined Screening Test in Pregnancy

Parents-to-be have a lot of choices to make during pregnancy, and one of the first is whether to have screening for chromosomal conditions. The combined screening test is now routine antenatal care throughout the UK. But how accurate is it, and what can the results really tell you?

In this article we’ll explain what you need to know about the accuracy of combined screening tests.

What Is the Combined Screening Test?

The combined screening test is a non-invasive test carried out between 11 weeks and 14 weeks of pregnancy. It looks at the risk of your baby having Down's syndrome (trisomy 21), Edwards' syndrome (trisomy 18), or Patau's syndrome (trisomy 13). It has two parts. The first is an ultrasound scan to measure the nuchal translucency. This is the area of fluid at the back of your baby's neck. 

The second is a blood test to measure two pregnancy hormones: pregnancy-associated plasma protein-A (PAPP-A) and free beta-human chorionic gonadotropin (β-hCG). These measurements are then analysed with other factors, such as your age, weight, ethnicity, whether you smoke, and medical history. It creates a personalised risk assessment, not a diagnosis.

Learn More About What Is Gynaecologists

Understanding Combined Screening Test Accuracy

When discussing accuracy, we need to look at two main measures: the detection rate and the false positive rate.

• Detection Rates for Chromosomal Conditions

Studies indicate that the combined test detects about 85–90% of affected pregnancies from Down's syndrome. For Edwards' syndrome and Patau's syndrome, the figures are 80–97% and 92% respectively. In one large prospective study of over 108,000 pregnancies, at a cut-off risk of 1 in 100, the test identified 90% of Down's syndrome, 97% of Edwards' syndrome and 92% of Patau's syndrome. These data indicate that the test is reliable when correctly carried out by appropriately trained staff.

• False Positive Rates

The false positive rate means how often the test predicts a higher chance when the baby is unaffected. The combined screening test has a false positive rate of around 3-5%.

That means, if 100 pregnant women get a higher chance result, about 95-97 of those women will not have an affected baby. This may sound like a high number, but remember that a higher chance result is not a diagnosis. It just means further testing may be recommended.

• What "Accuracy" Actually Means

Ok... That's a very important point. The combined screening test is a screening tool, not a diagnostic test and so only calculates probability and not certainty. If you get a result which says your chance is 1 in 50, it doesn't mean that you have a 1 in 50 chance (in a statistical medical sense) but rather that in a group of people with similar characteristics and the same test result, roughly 1 in 50 will have an affected pregnancy.

So, clinics like myGynaePlus go through these sorts of things in the consultation so parents understand what their individual result means in context.

Factors That Influence Test Accuracy

Several elements can affect how accurate your results are.

• Timing of the Test

The combined screening test works best within a specific timeframe. If performed too early or too late in pregnancy, accuracy decreases. The ideal window is 11 weeks and 2 days through to 14 weeks and 1 day of gestation.

Outside this window, different screening methods are offered, such as the quadruple test, which screens only for Down's syndrome and has slightly lower accuracy than the combined test.

• Quality of Ultrasound Measurement

The nuchal translucency measurement requires specialised training. Sonographers must follow strict protocols to obtain accurate measurements. Even small variations can affect the overall risk calculation.

Centres that perform high volumes of these scans typically achieve better accuracy because their staff maintain regular practice and expertise.

• Maternal Characteristics

Your personal characteristics factor into the calculation. Maternal age plays a significant role, as chromosomal conditions become more common with advancing maternal age.

Weight affects hormone concentrations in blood. Ethnicity can influence normal ranges for the biochemical markers. Smoking history and medical conditions like diabetes also contribute to the risk assessment.

How Results Are Interpreted

You'll receive your results within two weeks of having the test. The result comes as a single number, such as 1 in 200 or 1 in 1,500.

Lower Chance Results

Any result of 1 in 151 or higher (such as 1 in 500 or 1 in 2,000) is considered a lower chance result. This means your baby is unlikely to have one of the screened conditions. Most women receive lower chance results and require no further testing.

Higher Chance Results

A result between 1 in 2 and 1 in 150 is classified as higher chance. This doesn't mean your baby definitely has a chromosomal condition. It means you'll be offered additional testing options to get more information.

myGynaePlus and other specialist centres provide genetic counselling when higher chance results occur, helping you understand your options and decide whether to proceed with diagnostic testing.

What Happens After a Higher Chance Result?

If your combined screening test indicates a higher chance, you have several options.

• Non-Invasive Prenatal Testing (NIPT)

NIPT is a blood test that analyses fragments of placental DNA circulating in your bloodstream. This test has higher accuracy than the combined screening test, detecting over 99% of Down's syndrome cases with false positive rates below 1%.

NIPT has become increasingly available and is sometimes offered following a higher chance combined test result.

• Diagnostic Testing

Chorionic villus sampling (CVS) or amniocentesis can confirm whether your baby has a chromosomal condition. These tests examine actual foetal cells and provide definitive answers with over 99% accuracy.

The consideration with diagnostic testing is that both procedures carry a small risk of miscarriage, estimated at approximately 0.5-1%. Your healthcare provider will discuss these risks with you.

Limitations You Should Know About

Understanding what the combined screening test can and cannot do helps set realistic expectations.

• Not All Conditions Are Detected

The combined screening test specifically looks for Down's syndrome, Edwards' syndrome, and Patau's syndrome. It doesn't screen for other chromosomal conditions or genetic disorders.

Some structural abnormalities might be visible during the ultrasound component, but the test isn't designed as a comprehensive anatomy scan.

• False Negatives Do Occur

Approximately 10-15% of affected pregnancies will receive lower chance results. This means the test misses some cases.

If you have concerns about your pregnancy despite a lower chance result, discuss them with your midwife or obstetrician.

• Multiple Pregnancies

If you're expecting twins or more, the combined screening test can still be performed, but interpretation becomes more complex. The test evaluates overall risk rather than individual risk for each baby.

Comparing Alternative Screening Methods

Several other screening approaches exist, and understanding how they compare helps you make informed decisions.

The quadruple test, offered after 14 weeks, screens only for Down's syndrome using four blood markers. Research shows it has a detection rate of approximately 81%, lower than the combined test's 85-90%.

NIPT detects over 99% of Down's syndrome cases but costs significantly more. Many women choose the combined screening test first, then consider NIPT if they receive a higher chance result.

Some healthcare providers offer integrated screening, which combines first-trimester and second-trimester tests. This approach achieves detection rates above 95% but delays final results until the second trimester.

Making Your Decision About Screening

Screening is entirely optional. Some expectant parents choose not to have any antenatal screening, and this decision is respected within NHS care.

Consider what you would do with the information. Would a higher chance result lead you to seek diagnostic testing? Would you want to prepare differently if your baby has a condition? These personal questions matter more than statistics.

myGynaePlus offers consultations where you can discuss your individual circumstances, ask questions, and make the choice that feels right for your family.

What Current Research Shows

Recent studies continue to refine our understanding of combined screening test accuracy.

A 2016 study published in Ultrasound in Obstetrics and Gynaecology validated the first-trimester combined test across more than 108,000 pregnancies. The research confirmed detection rates of 90% for Down's syndrome at a 4% false positive rate.

Another study found that when combined screening is performed at 11 weeks of gestation, it outperforms second-trimester screening methods.

Ongoing research explores ways to improve accuracy further, including adding additional markers or combining screening with early anatomy assessments.

Questions to Ask Your Healthcare Provider

Before having the combined screening test, consider asking:

• What are the detection and false positive rates at your specific clinic?

• Who performs the nuchal translucency measurements, and what's their level of experience?

• What happens if my result shows a higher chance?

• What are my options if I decline this screening?

• How quickly will I receive my results?

Specialists at centres like myGynaePlus can answer these questions during your antenatal appointments.

The Role of Counselling and Support

It can be an emotional time to receive unexpected screening results. Access to genetic counselling can make a real difference to understanding your options.

Genetic counselors will explain what the numbers mean, go over the diagnostic testing procedure and support you no matter what decision you make.

Organisations like Antenatal Results and Choices offer helplines and information for parents.

Cost and Availability in the UK

The combined screening test is available free on the NHS to all pregnant women. The test is also available privately from clinics. Some offer quicker waiting times or more comprehensive services.

At myGynaePlus, antenatal screening tests are included as part of our antenatal care packages. Results are usually available within 1 week.

Looking at the Bigger Picture

In the past, women's only "screening test" was their age, so an offer to have a diagnostic test applied to all women of 35 and over. The offer of the combined screening test is a major step forward.

Screening is informative, but not diagnostic. If a woman has the test and wants to know more, then diagnostic tests that carry a risk of miscarriage may follow.

The screening test is not 100% accurate, but we know how accurate it is, as it has been researched very thoroughly. Most importantly it allows you to have discussions about what is most important to you.

Request For an Appointment Now!

Frequently Asked Questions

How accurate is the combined screening test compared to diagnostic tests?

The combined screening test detects approximately 85-90% of Down's syndrome cases with a 3-5% false positive rate. Diagnostic tests like amniocentesis detect over 99% of chromosomal conditions with near certainty. The key difference is that screening estimates probability while diagnostic testing provides definitive answers. Many expectant parents prefer starting with screening because it carries no miscarriage risk.

Can the combined screening test give false results?

Yes, like all screening tests, false positives and false negatives can occur. About 3-5% of women receive higher chance results despite having unaffected babies. Additionally, 10-15% of affected pregnancies receive lower chance results. This is why higher chance results lead to offers of diagnostic testing rather than being treated as final diagnoses.

What makes a combined screening test result higher or lower chance?

A result of 1 in 150 or higher risk (such as 1 in 50) is considered higher chance. Results of 1 in 151 or lower risk (such as 1 in 500) are considered lower chance. The cut-off point is somewhat arbitrary but represents where diagnostic testing is typically recommended. Your healthcare provider at clinics like myGynaePlus can explain what your specific result means for your situation.

Is the combined screening test better than NIPT?

NIPT has higher accuracy, detecting over 99% of Down's syndrome with lower false positive rates. The combined screening test remains widely used because it's offered free on the NHS, detects multiple conditions, and provides results earlier. Many women receive reassuring results from combined screening and don't need NIPT. NIPT is often used after higher chance combined screening results.