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More Diagnostic Tests FAQs
what are the risks?
The risk of miscarriage from CVS is about 1 -2 per cent. If miscarriage does occur, it usually takes place within a week of the test. In rare cases (about 1 in 100), an abnormality is seen in the placenta cells that is not found in the fetus. In this case, it may be necessary to perform another amniocentesis in order to confirm that the fetus is normal. In about 1 in 200 cases, no full culture result is obtained.
Amniocentesis
Amniocentesis is the removal of a small amount of the amniotic fluid that surrounds the fetus. Amniotic fluid contains skin cells from the baby that can be tested for chromosomal defects, for example, Down's syndrome.
When can it be performed?
The procedure is only performed after 15 weeks. Before 14 weeks, there is evidence of an increased risk of club feet (talipes) and the chances of causing a miscarriage are raised.
How is it performed?
As with CVS, the doctor performing the procedure will make sure that you fully understand the procedure and will obtain your written consent. You may not require an anaesthetic because the needle used for amniocentesis is finer than that used for CVS.
The procedure is conducted under ultrasound guidance and involves passing a needle through the skin and layers of your abdominal wall into the uterus, avoiding the fetus and, if possible, avoiding the placenta. A small sample of fluid (for example, 15 ml at 15 weeks) is then removed. Ifyou are Rhesus negative, you will be given an injection of Anti-D.
What is it used for?
Amniocentesis is mainly used to detect chromosome abnormalities (for example, Down's syndrome, or trisomy 21), genetic disorders (such as cystic fibrosis) and fetal infection. Amniocentesis is occasionally used late in pregnancy to assess whether your baby's lungs are mature enough for him to breathe on his own. Like CVS, the sex of the fetus can also be determined.
How quickly are the results available?
Some hospitals offer rapid tests in addition to the full culture test. These are usually known by their initials -fish or PCR (fluorescence in situ hybridization or polymerase chain reaction). These results are available with 2-3 days of the procedure and provide specific information, for example, whether or not the fetus has Down's syndrome.The full culture results take longer (usually about 2 weeks).
Is it painful?
As with CVS, many women find the procedure uncomfortable but not too painful. Amniocentesis usually takes only about 10 minutes and is thus well tolerated by most women who have it.
What are the risks?
The risk of miscarriage after amniocentesis is about 1 per cent and it usually occurs within a week of the procedure. In about 1 in 200 cases, no full culture result is obtained, in which case the FISH and PCR results are usually reliable. Otherwise, it may be necessary to repeat the test.
Fetal blood sampling
Another method of diagnostic testing is sampling the fetal blood, either from the cord (cordocentesis) or from a vein in the fetal liver (intrahepatic vein). The technique is similar to amniocentesis and CVS except that the aim is to obtain a small sample of fetal blood rather than amniotic fluid or placental tissue.
When can it be performed?
Fetal blood sampling is usually performed after 18-20 weeks of pregnancy.
How is it performed?
For cordocentesis, ultrasound is used to locate the point where the umbilical cord inserts into the placenta. The procedure then involves inserting a fine needle through the abdomen and uterine walls into the umbilical cord, using ultrasound as a guide. Once the fetal blood sample has been obtained, it is sent to the laboratory for analysis. A full chromosome result is usually available within 72 hours.
Blood samples from the intrahepatic vein are taken in a similar way, except that the needle is guided into the fetal liver.
What is it used for?
Fetal blood sampling was formerly used to obtain a full chromosome analysis of the fetus, which took just a few days. However, since the advent of rapid testing of amniotic fluid (by PCR or FISH, see above), it has been used less often for this purpose because amniocentesis is technically less demanding.
Instead, fetal blood sampling is now mainly reserved for conditions such as fetal anaemia (for example as a result of Rhesus disease), fetal infection (for example toxoplasmosis or rubella) or fetal blood disorders (such as sickle-cell disease). If fetal anaemia is diagnosed, the fetus can be given a blood transfusion using the same needle.
what are the risks?
The major risk of fetal blood sampling is a 1-2 percent risk of miscarriage.
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The future: non-invasive diagnostic tests In the not-too-distant future, the use of CVS and amniocentesis may largely be replaced by a blood test on the mother. The main advantage of this method of testing is that it is non-invasive and carries no risk of miscarriage. Research has shown that it is possible to identify fetal cells as well as the fetal DNA that normally circulates in the mother's blood. The challenge is to accurately distinguish between the components derived from the fetus and those of the mother. It is already possible to determine fetal gender and blood group in this way. |