Down syndrome (also referred to as Down’s syndrome) can sometimes be missed during pregnancy. Screening tests assess the chance that a fetus has the condition, but they cannot identify every pregnancy affected by Down syndrome. Only diagnostic testing can confirm the condition before birth.

A missed diagnosis does not automatically mean that medical negligence occurred. However, a wrongful birth claim may be possible if negligent screening, testing, advice or communication deprived parents of the opportunity to make an informed decision about whether to continue the pregnancy.

In this guide, we explain how antenatal screening works, why Down syndrome may not be identified and when failures in care could support a medical negligence claim.

What is Down syndrome?

Down syndrome is a genetic condition caused by an extra copy of chromosome 21 in all or some of the body’s cells. It is also known as trisomy 21.

There are three main forms:

• Full trisomy 21: every cell has an extra copy of chromosome 21.
• Mosaic Down syndrome: only some cells have the extra chromosome.
• Translocation Down syndrome: additional genetic material from chromosome 21 attaches to another chromosome.

Down syndrome affects people differently. It can influence learning, physical development and health, but each person has their own abilities and needs. The physical features associated with Down syndrome can also vary considerably between individuals. Some people also have congenital heart conditions or require support with hearing, vision and other aspects of their health.

How is Down syndrome screened for during pregnancy?

In England, eligible pregnant women should be offered screening for Down syndrome, Edwards’ syndrome and Patau’s syndrome as part of their antenatal care. The NHS screening programme emphasises that parents should receive enough information to make a personal, informed choice about whether to accept it.

The combined test

The combined test is normally offered during the first trimester. It includes:

• A blood test, usually taken between 10 and 14 weeks.
• An ultrasound scan measuring the fluid at the back of the fetus’s neck, known as nuchal translucency.
• Factors including maternal age.

These results are combined to calculate the chance of the fetus having Down syndrome, Edwards’ syndrome or Patau’s syndrome.

The result is described as higher chance or lower chance. It is not a diagnosis. The NHS screening programme sets a detection-rate standard of at least 85 per cent for Down syndrome using first-trimester combined screening, with current modelled performance at around 87 per cent. This means that some affected pregnancies will still receive a lower-chance screening result.

The chance of a pregnancy being affected by Down syndrome also increases with maternal age. The increase becomes more marked as women get older, particularly from their mid-30s onwards. For context, current NHS screening guidance gives women aged 40 a chance of around 1 in 100 of having a baby with Down syndrome at 16 weeks of pregnancy.

More detail on the screening pathway, including the role of maternal age and the different markers used, is available in the NHS Fetal Anomaly Screening Programme guidance.

The quadruple test

If the combined test cannot be completed, for example because the pregnancy is too advanced or the nuchal translucency measurement cannot be obtained, the quadruple screening test may be offered.

This is a blood test generally carried out between 14 and 20 weeks of pregnancy. It assesses the chance of Down syndrome but does not provide a definite diagnosis. The combined and quadruple screening tests use different markers, and the quadruple test is less accurate than the combined test.

Non-invasive prenatal testing

Non-invasive prenatal testing, usually called NIPT, analyses fragments of placental DNA in the mother’s blood. The test can be performed from 10 weeks of pregnancy, although within the NHS screening pathway in England it is generally offered following a higher-chance combined or quadruple screening result.

NIPT is more accurate than the initial screening tests. For Down syndrome, some validated NIPT tests have detection rates of more than 99 per cent, meaning false-negative results occur in fewer than 1 per cent of affected pregnancies. However, NIPT is still a screening test rather than a definite diagnosis.

A higher-chance NIPT result should therefore be discussed alongside the option of diagnostic testing.

Which tests can confirm Down syndrome?

Diagnostic tests examine genetic material and can confirm whether the fetus has Down syndrome.

Chorionic villus sampling

Chorionic villus sampling, commonly known as CVS, involves taking a small sample of cells from the placenta. These cells usually have the same genetic material as the fetus.

CVS is an invasive procedure and carries a small risk of miscarriage. NHS guidance on CVS estimates the chance at less than 1 in 200 for most singleton pregnancies.

Amniocentesis

Amniocentesis involves taking a small sample of amniotic fluid from around the fetus. This contains cells that can be tested for chromosomal conditions.

Amniocentesis is usually carried out from 15 weeks of pregnancy. Results for Down syndrome, Edwards’ syndrome and Patau’s syndrome are generally available in around three working days. Where testing is being carried out for other conditions, more detailed results can take around two weeks.

Both procedures are invasive and carry risks that should be explained before parents decide whether to proceed. NHS guidance states that prenatal diagnosis should be offered following a higher-chance screening result or unexpected structural findings during the 20-week scan.

Parents should receive clear and balanced information about:

• What the result means
• The available tests
• The purpose and limitations of each test
• The possible risks to the pregnancy
• When results are likely to be available
• The choices available following a diagnosis

Receiving a chance of your baby having Down syndrome during pregnancy can involve significant decisions for parents. Emotional support is therefore an important part of good care. Many parents find comfort in sharing their experiences with other parents, and support groups can provide an opportunity to connect with families who have faced similar questions and decisions.

Can Down syndrome be missed on an ultrasound?

An ultrasound scan cannot rule out Down syndrome.

Certain findings may be associated with the condition, including increased nuchal translucency, some heart conditions and differences in the development of particular structures. However, these findings are not present in every fetus with Down syndrome and may also occur in pregnancies where the fetus does not have the condition.

The 20-week scan primarily checks for specified physical conditions and structural differences. A fetus with Down syndrome may have no obvious findings on that scan. The absence of visible differences does not provide a definitive answer.

A claim may nevertheless warrant investigation if a sonographer failed to identify or report a finding that should reasonably have led to further assessment.

Why might Down syndrome be missed?

There are several possible explanations, not all of which involve medical negligence.

A false-negative screening result

Screening tests cannot detect every pregnancy affected by Down syndrome. A properly performed test may return a lower-chance result even though the fetus has the condition.

This applies even to more accurate forms of screening. NIPT has a very low false-negative rate for Down syndrome, but it cannot completely eliminate the possibility of an affected pregnancy receiving a lower-chance result.

Screening was not offered

A midwife or doctor may fail to explain or offer screening within the appropriate time frame. This can remove the opportunity to decide whether to undergo screening and any later diagnostic testing.

Incorrect ultrasound measurements

The nuchal translucency measurement must be taken and recorded accurately. Errors could affect the calculated screening result.

Laboratory or calculation errors

A blood sample may be labelled, processed or analysed incorrectly. Relevant information could also be entered inaccurately when calculating the screening result.

Maternal age is one of the factors used in the combined screening calculation because the chance of Down syndrome increases as maternal age rises. This makes it important that information used in the calculation is recorded accurately.

Results were not communicated

A higher-chance result may not be passed to the parents promptly, or the options for NIPT and diagnostic testing may not be explained.

Further tests were not arranged

Unexpected findings or a higher-chance screening result may require referral to an antenatal screening or fetal medicine team. A failure to arrange timely follow-up could prevent parents from receiving further results that may show a high chance of Down syndrome

Can mosaic Down syndrome be harder to identify?

Mosaic Down syndrome occurs when only some cells contain the extra copy of chromosome 21. Its features can be less pronounced, although this varies considerably between people.

Because the proportion of affected cells can vary, mosaic Down syndrome may sometimes be harder to identify through physical examination or particular samples. Genetic testing may be required to confirm the diagnosis.

This does not mean every delayed diagnosis of mosaic Down syndrome involves negligence. The care must be assessed against what a reasonably competent clinical team should have recognised and done at the time.

When could a missed diagnosis lead to a wrongful birth claim?

A wrongful birth claim may be possible where negligent antenatal care prevented parents from receiving an accurate diagnosis or appropriate information in time.

Examples may include:

• Failure to offer antenatal screening
• Incorrect performance of the combined test
• Errors involving a blood sample or screening calculation
• Failure to recognise significant ultrasound findings
• Misreporting a screening or diagnostic result
• Failure to communicate a higher-chance result
• Failure to discuss NIPT, CVS or amniocentesis
• An avoidable delay in referral to fetal medicine specialists

The claim is not based on suggesting that a person with Down syndrome has less value or loved any less. It concerns the parents’ loss of the ability to make an informed decision about the pregnancy and the additional needs arising from the condition.

You can read more about the circumstances that may lead to a wrongful or unlawful birth claim.

What must be proven?

A wrongful birth claim requires evidence of two connected legal issues.

Breach of duty

The care provided by a midwife, sonographer, obstetrician, laboratory or hospital fell below the reasonable standard expected.

Causation

The parents must show that, with reasonable care, they would have received the correct information in time and would probably have made a different decision about the pregnancy.

This requires a careful and sensitive assessment of the medical records, the available options at the time and the parents’ own evidence.

JMW's guide to how to prove medical negligence explains these legal tests in more detail.

What evidence may be needed?

Relevant evidence may include:

• Antenatal and maternity records
• Screening consent records
• Blood test results and laboratory data
• Ultrasound images and reports
• Nuchal translucency measurements
• NIPT results
• CVS or amniocentesis records
• Referral letters and fetal medicine notes
• Records of conversations with doctors and midwives
• Detailed accounts from the parents

We will instruct independent obstetric, fetal medicine, ultrasound or genetics experts to assess whether the care met a reasonable standard and whether different care would probably have provided a diagnosis in time.

For more information about how evidence fits into an investigation, read JMW's guide to the stages of a medical negligence claim.