Chorionic villous sampling (CVS) and amniocentesis are two similar tests, performed during pregnancy to determine your baby’s chromosome arrangement. At SHORE FOR WOMEN we have performed hundreds of both CVS and amniocentesis procedures. Most women do not need a CVS or amniocentesis during their pregnancy – these tests are usually reserved for couples whose baby has a higher than average risk of a chromosomal problem:
Technically, both tests are very similar, involving a quick, 5-10 minute out-patient procedure by a fetal medicine specialist using ultrasound guidance. During amniocentesis, a very thin needle is inserted through the woman’s abdomen into the amniotic fluid (waters) surrounding the baby. A small amount of fluid (20ml) is removed and sent to the laboratory for analysis. With a CVS, the sample is obtained from the placenta (which can lie in different positions). CVS is usually performed through the abdomen and occasionally, through the cervix (like having a pap smear).
Both tests involve inserting a needle into the pregnancy and therefore they carry a small risk of miscarriage. This risk is approximately 1 in 200 (or 0.5%). Our procedure-related risk of miscarriage after these tests at SHORE FOR WOMEN is comparable with the best centres internationally. In rare cases, CVS or amniocentesis can lead to mixing of the maternal and fetal blood. For this reason, women whose blood group is Rh-D negative should receive an anti-D injection after the test.
The 2 tests provide the same information and results. However, CVS can be performed earlier in pregnancy (after 11 weeks) whereas women must be at least 15 weeks before amniocentesis can be performed. This means that women who want testing prior to 15 weeks can be offered a CVS. Some older studies suggest that the miscarriage risk following CVS is slightly higher, though this probably reflects the fact that CVS is performed earlier in pregnancy.
The sample obtained is analysed in two ways. Half of the sample is processed through a “quick” test (called a FISH test) which takes 2 working days, designed specifically to look for the most common chromosomal problems, including Down syndrome. The other half of the sample undergoes a full chromosome analysis, which takes 2-3 weeks. In a very small number of cases (<1%), the cells fail to grow in the laboratory and a repeat test is necessary.
CVS / amniocentesis is the “gold standard” for prenatal diagnosis and the results are effectively 100% (99.99%) with regards to a baby’s chromosomes. If the full result shows that your baby does (or does not) have, for example, Down syndrome, then you can be certain that is the case. It is important to realise that these tests cannot test for every genetic syndrome (each chromosome carries thousands of genes). Thus, while a normal result excludes chromosome problems, this is not the same as saying that your baby will be “genetically-perfect”.
Some couples want certainty, even if the chance of a chromosomal problem for their baby is very low. The only way to achieve this is with a CVS or amniocentesis. In the United States, women aged over 35 years are routinely offered amniocentesis. This is not the case in Australia – however, if a couple feel strongly that they wish to undergo testing then of course we will support this.
Prenatal microarray is a newer laboratory method for analysing the sample obtained during CVS or amniocentesis. Unlike conventional karyotype, which detects chromosomal abnormalities under the microscope, microarray examines the chromosome at the molecular level and so is more likely to detect smaller genetic abnormalities. However, microarray also detects “variants of unknown significance” in 1-2% of tests. These are genetic differences which may or may not cause problems for the child – this difficult scenario can create huge anxiety for couples. At SHORE FOR WOMEN, our current belief is that prenatal microarray should be reserved for pregnancies where there is suspicion of an unusual genetic syndrome.