Diagnostic tests available during pregnancy

Chorionic villus sampling (CVS)

This is a 'diagnostic test' and one advantage of the CVS test over amniocentesis is that it can be carried out early in pregnancy, at about 11-13 weeks. At present this test isn't available everywhere, but can show up a number of disorders in your developing baby, including Down syndrome, though not spina bifida. CVS is 99.9% accurate, but is carried out less often than amniocentesis, due to the fact that it is more specialised and is also expensive. In Europe, both CVS and amniocentesis are carried out more where there is private and insurance-based medical care.

At present, CVS and amniocentesis are the only tests available which can make a definite diagnosis of disabling conditions in the unborn baby.

How is the CVS test carried out?

Cells from the developing placenta are drawn off by aspiration (a fine needle is inserted into the placenta to obtain a small tissue biopsy) either through the abdomen, or vagina and cervix. The test is described as "uncomfortable", rather than painful, but only takes about 5-10 minutes. Results can be received back within 7-10 days, although sometimes they can be inconclusive, meaning that a re-rest is needed. You may be offered this test if you are an older woman who is at higher risk of having a baby with Down syndrome, or if you are at risk of having a baby with certain inherited diseases, such as sickle cell anaemia or thalassaemia. The risk of miscarriage with this test is somewhat higher than for amniocentesis, at about 1.5 - 2%. As with amniocentesis you need to balance this risk against the value of the test to you. Professor Nicolaides of the Harris Birthright Research Centre believes that if the procedure is carried out by trained doctors the miscarriage rate is the same or lower than that of amniocentesis and that hospitals that carry out CVS most often are likely to produce the best results.

Latest research

A new, faster, method of analysis from CVS samples called Fluorescent in Situ Hybridisation (FISH) should soon to be available.


Alpha-feto-protein (AFP)

This 'diagnostic test' is performed at about 15-20 weeks into pregnancy to find out the level of alpha-feto-protein, which is a protein from the baby, found in the mother's blood. If the level is higher or lower than normal it may be an indication that the baby might be at risk from either spina bifida, Down syndrome or other neural tube defects (problems with the development of the spinal column, spinal cord, brain, or skull as in spina bifida or anencephaly). If the test necessitates, a second AFP test may be given, then an ultrasound and possibly amniocentesis (see below) to give more detailed information. Some hospitals test routinely for alpha-feto-protein, while others don't. If you want to have the test and it is not offered it is wise to ask for it well in advance.

What does a test for AFP entail?

The specific details of the procedure may vary, but blood is usually taken from the mother between the 15th and 18th week of pregnancy and sent off for analysis.



This is a 'diagnostic test', which takes a sample of amniotic fluid from the uterus containing cells from the baby's system. It is a usually quick and painless test that may be offered at about 16-19 weeks of pregnancy, especially if you are over 35 and have a higher risk of having a baby with Down syndrome. It is also offered if your family history suggests there may be a risk of your baby having muscular dystrophy, haemophilia, cystic fibrosis or other genetic disorders as well. Again, it may be offered if the AFP test (see above) shows a risk of Down syndrome, or spina bifida. In the latter, a detailed ultrasound scan may provide enough information instead. Amniocentesis is 99.9% accurate in diagnosis of Down syndrome, other genetic abnormalities and spina bifida.

What happens during an amniocentis?

Using ultrasound to guide the procedure, a long, thin, needle is inserted through the mother's abdomen into the amniotic sac and a small sample of amniotic fluid is extracted for examination. You will be advised to rest for a day or two afterwards to minimise the risk of miscarriage.

Be aware that a result can take up to three or four weeks and if it comes back as positive, you are halfway or even further into pregnancy should you make a decision to terminate.

Amniocentesis does carry a small risk of miscarriage - according to the Health Education Authority, that risk is around 1%. Therefore you need to balance this against the value of the test to you. Of course a normal test is no absolute guarantee that there will not be any problems, but is nonetheless reassuring.



This is a 'diagnostic test', that is also known as foetal blood sampling or umbilical vein sampling. It is used to diagnose Down syndrome where, for example, earlier tests have shown a possible problem. It will need to be carried out at a regional specialist foetal medicine centre, since it is a very specialised procedure.

Cordocentesis can also detect infection with diseases such as toxoplasmosis, which can be caught from cat faeces, raw and undercooked meat, unwashed fruit and vegetables, raw eggs, unpasteurised products, the soil and contact with sheep at lambing time. Infection with toxoplasmosis can lead to miscarriage, stillbirth, or survival with growth problems, blindness, water on the brain (hydrocephalus), brain damage, epilepsy, or deafness (if the infection was recent enough to pose a risk to the unborn foetus, antibiotics are given). Additinally, Cordocentesis is used to detect rubella infection, as well as to perform a blood count on a foetus that is suspected of having anaemia.

What does a cordocentesis test entail?

From 18 weeks onwards the baby's blood is constantly examined using a sample of blood carefully extracted from the umbilical cord. The test is carried out in a similar way to that of amniocentesis, though results are available within 72 hours.

What are the risks associated with cordocentesis?

The risk of miscarriage is reported as 1-2%.


Ultrasound Scans

Introduced in the late 1950s, ultrasound is a special test using high-frequency sound waves, that is carried out by a specially trained person (sonographer) using a special machine. It shows a continous picture of the moving foetus on a monitor screen and you are usally given the option of purchasing a print/s of this.

It is non-invasive, accurate and considered risk-free, taking approximately 15 minutes - 1 hour, with preliminary results being available immediately. The only discomfort might be that during early pregnancy you are required to have a full bladder in order for the test to work, which can be uncomfortable!

Ultrasound is used to establish the age and number of foetuses being carried, their size and health, the location of the placenta, amount of amniotic fluid, and to detect possible abnormalities in the foetus(es) - such as the rare condition of anencephaly - and mother's pelvis. It can reveal foetal malformations and severe disorders such as spina bifida (a detailed ultrasound scan picks up most cases of spina bifida when carried out at about 18 weeks into pregnancy), as well as detect the risk of inheriting a sex-linked disease. It can also be used to determine the sex of the baby, although this is not 100% accurate. Ultrasound may be performed to monitor foetal growth in late pregnancy and be used to assist in other procedures, such as amniocentesis, CVS, and foetal blood testing, where a needle is required to be inserted in the uterus. Before the scan the pregnant mother will be asked to drink as much she can to fill up the bladder and thus push the womb out of the pelvis so that the baby is more visible. The person doing the scan (sonographer) puts a clear gel onto the mother's stomach to improve contact between the transducer (handpiece used on the stomach) and skin and moves the transducer around the abdomen to get the best picture. It is then possible to see images of the baby on a special video screen.

When is ultrasound carried out?

The first is at around 12 weeks. Some obstetricians say that all women should also have a scan to check the health and growth of the foetus between 30-32 weeks. Some offer scans to women who have had problems in previous pregnancies e.g. miscarriage, diabetes, stillbirth, pre-eclampsia, retarded growth. Also women who develop complications during the pregnancy may be offered a scan.

Who is offered it?

All pregnant women are offered a dating ultrasound scan. Around 90% are also offered a scan which looks for major physical abnormalities, such as Down syndrome, spina bifida, or hydrocephalus. This is similar to a dating scan, but requires greater expertise by the sonographer. Ultrasound can also be used to look for ectopic pregnancy and miscarriage.

What are the advantages of ultrasound scans?

Ultrasound scans are effective, non-invasive and can reassure parents about the health of their unborn child. They are accurate at dating and give parents the opportunity to pick up any major physical abnormalities (see above). They also allow parents to see their unborn child in the womb.

Are there any disadvantages?

Only after 18 weeks are scans likely to show anything but major problems with the baby and they can sometimes falsely identify problems. They also rely on the expertise of the sonographer, which can vary from hospital to hospital but, in general, foetal medicine centres and hospitals attached to universities are the best equipped and have the most well-trained staff.

Results of a Scandinavian study showed that mothers who had a lot of ultrasound scans were more likely to produce a left-handed baby, making some scientists think that the high-frequency sound waves produced during ultrasound somehow made changes to the unborn baby's brain. However, there is very little other evidence of these scans affecting a baby.


Screening Tests

Read about the various screening tests available during pregnancy.


NHS Direct - 24-hour nurse-led helpline with confidential healthcare advice and information
List of NHS Trusts
NetDoctor.co.uk - information on children's health & diseases
The Fetal Medicine Foundation
Harris Birthright Research Centre (originators of nuchal fold scan)
Medical Dictionary