What are antenatal diagnostic tests?
Diagnostic tests are used to provide fetal diagnoses antenatally and are only offered to those women who are at risk of having a fetus with a congenital abnormality. Screening tests differ: they are offered to all pregnant women and include:
- Blood tests to look for infections (Hepatitis B, HIV or syphilis), abnormalities (Sickle Cell Disease, Thalassaemia) and Rhesus status
- Down’s screening (serum screening, nuchal translucency or a combination, depending on gestation)
- An anomaly scan between 18 and 20 weeks gestation.
Screening tests do not carry any risk to the pregnancy; they are performed for conditions which have a reasonably high prevalence and are implemented in order to identify the ‘high risk’ pregnancies. Any pregnancy identified as ‘high risk’ is then offered further, 'diagnostic' testing. Diagnostic tests are usually invasive and thus carry some risks, including an increased risk of miscarriage. The antenatal diagnostic tests most commonly used are amniocentesis and chorionic villus sampling.
Who gets offered an antenatal diagnostic test?
Any pregnancy that is ‘at risk’ of a fetal abnormality is offered an antenatal diagnostic test. This is either women who are identified as ‘high risk’ from screening or those who are at risk for other reasons, such as they have had a previous pregnancy with a fetal abnormality or there is an inherited condition that runs in the family.
About 5% of pregnant women get offered an antenatal diagnostic test.
Why are antenatal diagnostic tests performed?
Antenatal diagnostic tests are performed in order to give a diagnosis of a congenital abnormality. This allows the parents to make an informed decision regarding the pregnancy. They may wish to choose to terminate the pregnancy. It also allows the delivery to be suitably planned and any postnatal care to be arranged in advance.
What antenatal tests are available?
Under ultrasound guidance (preferably continuous) a thin needle (20-gauge is maximum) is passed transabdominally into the amniotic cavity and a small amount of amniotic fluid (10-20mL) is removed. The amniotic fluid contains fetal skin fibroblasts, which can then be tested for abnormalities. The majority (96%) of amniocenteses are performed without local anaesthetic. The procedure has been likened to that of venepuncture in terms of discomfort.
Amniocentesis is performed from 15 weeks’ (15+0) gestation. The procedure is associated with a small risk (about 1%) of miscarriage, but is over 99% accurate.
Early amniocentesis (before 15 weeks’ gestation) is not recommended as it is associated with a higher miscarriage rate, fetal talipes and respiratory morbidity.
Chorionic villus sampling:
Under ultrasound guidance, a thin needle is passed into the chorionic plate of the placenta. This procedure can either be performed transcervically or transabdominally. Both methods result in obtaining a sample of chorionic villi which is fetoplacental in origin. This sample can then be tested for fetal abnormalities. 98% of chorionic villus sampling procedures are performed under local anaesthetic.
Chorionic villus sampling is usually performed between 11 weeks and 0 days and 13 weeks and 6 days’ gestation. It, like amniocentesis is also associated with a risk of miscarriage. The transcervical method is thought to have a slightly higher risk than that of transabdominal sampling. Overall the risk is between 1-2%. It is thought to be higher than amniocentesis because the risk of spontaneous miscarriage is higher at this earlier gestation.
Chorionic villus sampling is between 97.5-99.6% accurate.
Whilst amniocentesis and chorionic villus sampling are the most widely accessible antenatal diagnostic tests, other tests are also available, such as cordocentesis. This involves obtaining a sample of fetal blood from the umbilical cord; this procedure is usually carried out from 20 weeks’ gestation and is associated with a 1% risk of miscarriage.
All the diagnostic tests described above also carry a small chance of other risks. These include amniotic fluid leakage, maternal Rhesus status sensitisation (therefore need to give prophylactic anti-D to all Rhesus negative women undergoing invasive diagnostic procedures) and infection, which can very rarely lead to sepsis (0.1%).
When counselling for these procedures it is important to:
- Explain the purpose of doing the procedure
- Explain its limitations
- Explain the risks involved
- Reassure that it is relatively painless
- Allow the patient to make an informed decision
How are the samples analysed?
The samples are sent off to the laboratory and are analysed in the following ways:
- the results are available within 24-72 hours
- identifies trisomies (21, 13, 16), Turner’s and Klinefelter’s syndromes
- takes 2 weeks for the result
- provides full fetal karyotype
What does the future hold?
There is currently research into non-invasive tests that could be used for antenatal diagnosis, for example PCR techniques that will allow fetal DNA analysis from a maternal blood sample.
Impey L, Child T. Obstetrics and Gynaecology. 3rd edition. Wiley-Blackwell; 2008. pp. 175-77
Campbell S, Lees C. Obstetrics by Ten Teachers. 17th edition. New York: Oxford University Press; 2000. pp. 148-151
http://www.rcog.org.uk/files/rcog-corp/GT8Amniocentesis0111.pdf Accessed on 17/04/11
http://www.nice.org.uk/CG062 Accessed on 17/04/11