If the result of a screening test indicates that your baby is at high risk of abnormality, you will be offered a follow-up diagnostic test, for example, chorionic villus sampling (CVS) or amniocentesis. These tests are not routinely available or performed because they are invasive, and there is a risk of provoking a miscarriage in 1 -2 per cent of cases.
The decision to undergo diagnostic testing is therefore often difficult and causes a huge amount of anxiety. The midwife or obstetrician is there to ensure that you fully understand the benefits and risks of any test. Remember that no one has to have a diagnostic test. The final decision has to be made by the parents. who is offered a diagnostic test?
A diagnostic test is offered to women who have an increased risk of having a baby with a chromosomal abnormality. The risk factors include:
- Late age (that is, late 30s and 40s)
- A history of a chromosome abnormality (for example, a previous child with Down's syndrome)
- A family history of a genetic disorder like cystic fibrosis
- A problem seen on the ultrasound sound scan
- An increased serum screening risk or a raised nuchal translucency.
If the test result is abnormal, you will be invited to discuss the available options with your obstetrician. It is important to remember that a normal result, although reassuring, does not rule out the possibility of there being other problems.chorionic villus sampling (CVS)
CVS is the removal and examination of a small amount of placental tissue (afterbirth), known as chorionic villi. Because the placenta and fetus are made up of the same cells, an analysis of the cells of the placental tissue provides information about the fetus itself. Analysis of the chromosomes in these cells will detect any abnormalities, while analysis of the individual genes will reveal any genetic defects. when can it be performed? CVS is only performed after 11 weeks because some evidence suggests that CVS performed before 10 weeks can result in limb deformities.
How is it performed?
Before performing the procedure, your doctor will discuss it with you fully and will usually obtain your written consent. CVS involves injecting a local anaesthetic and then, under ultrasound guidance, passing a needle through the skin and layers of your abdomen into the placenta. A sample of the placenta is then taken and sent for analysis. Occasionally the procedure is performed through the cervix, for example, if the placenta is very low or the uterus is tilted back (retroverted). If you are Rhesus negative, you will be given an injection of Anti-D.
What is it used for?
CVS detects chromosome abnormalities (for example, Down's syndrome) and genetic disorders (for example, muscular dystrophy, Huntingdon's chorea, sickle-cell anaemia and cystic fibrosis). The sex of the baby can also be determined in this test.
How quickly are the results available?
An initial result is usually available within 2-3 days.The full culture result (obtained from a culture of the cells) takes about 2 weeks.
Does it hurt?
Most women who have had a CVS describe the procedure as uncomfortable but not too painful. Afterwards, it is normal to experience some abdominal discomfort, which can be relieved by mild painkillers, such as paracetamol. If the symptoms worsen or any bleeding occurs after this test, you should contact your doctor urgently.