Antenatal Screening

Deciding whether to have further diagnostic tests

When screening results suggest the baby may have a particular condition, further investigations will be offered to try to give a definite diagnosis.

Suspected heart conditions and neural tube defects (such as spina bifida) may be investigated by further specialised scans. As these are non-invasive tests which carry no risk, all the people we spoke to in this situation decided to have the further scans. One woman was offered a cardiac scan because she was diabetic and therefore at increased risk of having a baby with a heart condition.

Chromosomal conditions including Down's syndrome can be detected by chorionic villus sampling (CVS) from 10 weeks in pregnancy, and amniocentesis from around 15 weeks. CVS takes a sample of placenta for analysis, whilst amniocentesis takes a sample of the fluid round the baby. Amniocentesis can also test for neural tube defects.

It usually takes 2-3 weeks to get the full results from these tests. However, most centres offer quick results within 3 working days, using the QF-PCR test (which provide a rapid diagnosis of the most common chromosome abnormalities including Down's syndrome, Edwards' or Patau's syndrome). It is important to remember that even though you may be reassured that the baby has not got Down's syndrome, very occasionally another chromosomal condition is found later following full examination of all the chromosomes.

Rarely, a test called cordocentesis may be carried out later in pregnancy (after 18 weeks). This takes a sample from the umbilical cord, and can provide fast results, but has a miscarriage risk of 1-2%.

Both amniocentesis and CVS have a risk of miscarriage of around 1%, so people think very carefully before going ahead. We spoke to two couples - both with a previous history of miscarriages - who were told they were at high-risk after the nuchal scan. One decided the risk of another miscarriage was too high and chose not to have amniocentesis. It turned out the baby did not have Down's syndrome. The second couple felt they needed certainty and decided to have CVS. Although the baby did not have Down's syndrome, a cardiac scan offered at the same time as the CVS detected a heart condition.

Even though it was hard to grasp what was happening to them, many couples saw further tests as the only sensible option, and felt comfortable about staff advising them what they should do next. Some were advised that in the hands of expert staff the miscarriage risk is lower than the national rate, which helped them decide.

Occasionally, people felt a bit pressurised, and one woman felt perhaps she should have waited a few days to think about it. Some people were unhappy if it was assumed in subsequent pregnancies they would want the same tests again. (See also 'Making the decision to end the pregnancy' and 'Views on screening and feelings in later pregnancies').

Some people felt they already had enough information without further tests. For example, a woman who had already decided to end the pregnancy because of a serious heart condition felt she did not need to know if the baby also had Down's syndrome.

A couple who decided they wanted to continue the pregnancy also decided against amniocentesis. They briefly had second thoughts till their GP helped them think it through. On the other hand, a woman who had made up her mind to go ahead after being told her baby had a serious heart condition still chose amniocentesis.

Although nearly everyone felt they had been well advised about a difficult decision, one woman was angry to have no-one there to discuss the results with her that day. She was also unhappy with the information given by the consultant she saw next day, although things improved when she saw a specialist. She needed accurate information to inform her decision. She felt much better cared for at the specialist hospital, and the professor of fetal medicine made a point of involving her partner.

Several people were influenced by watching TV programmes. This prompted one woman who had a termination in her first pregnancy to have amniocentesis in her second one. It also made another woman decide to have amniocentesis in her first pregnancy instead of screening, even though the doctor performing the amniocentesis queried whether it was necessary. She discovered her baby had Down's syndrome. Another woman's family history made her choose amniocentesis despite appearing to be at low-risk.

A few people were offered diagnostic tests instead of screening tests. Having missed the deadline for a combined nuchal scan and blood test, one woman was offered CVS but refused. In America, a woman was strongly advised to have amniocentesis as a routine investigation, but she did not agree.



Non Invasive Prenatal Test (NIPT) is a new screening test that can be used to detect Down's syndrome. It is a non-invasive test using a maternal blood sample to test free-cell fetal DNA. During pregnancy, fragments of DNA from the placenta called free-cell fetal DNA circulate in the mother’s blood steam. These DNA fragments (CFFDNA) can be genetically tested along with the mother’s blood to show if there is an increase in the number of abnormal chromosomes. This screening test is not currently offered by the NHS, however some NHS Trusts have piloted the test and a number of maternity units do offer testing privately. The sample has to be sent outside the UK for analysis so the results take about two weeks to come back. The cost of the test varies between clinics (£350 to £550). Large scale studies show that the test has a detection rate of over 99% (ARC July 2017) and also tests for some other common chromosomal anomalies; Edwards syndrome (trisomy 18), Patau syndrome (trisomy 13) and Turner syndrome. It can be performed from 10 weeks of pregnancy and unlike amniocentesis and CVS has no risk of miscarriage. However if the NIPT gives a positive result, an amniocentesis is required for confirmation. For more information see our pregnancy resources.

Last reviewed July 2017.
Last updated July 2017.

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