Pregnancy—Chorionic villus sampling (CVS) and amniocentesis
Chorionic villus sampling (CVS) and amniocentesis are tests performed in pregnancy to identify chromosomal abnormalities such as Down syndrome or genetic conditions such as cystic fibrosis.
Prior to having a CVS or an amniocentesis it is important that you have a good understanding of the risks and limitations of each procedure.
This brochure will provide some information for you to consider but it is not intended to take the place of comprehensive counselling available at the Mater Centre for Maternal Fetal Medicine.
Reasons for having CVS or amniocentesis
Women may choose to have a CVS or an amniocentesis if:
- the results from their first or second trimester Down syndrome screening test showed the baby to be at high risk for a chromosomal abnormality
- they require certainty regarding the diagnosis of Down syndrome or another chromosomal abnormality
- an ultrasound during their pregnancy has identified an abnormality in the baby
- they have a known family history of a genetic disorder.
Chorionic villus sampling (CVS)
CVS is performed at 10 to 14 weeks of pregnancy by taking a small sample of placental tissue (chorionic villi). Cells found in the chorionic villi almost always have the same chromosomes as the baby. Therefore abnormalities found in the chromosomes of the chorionic villi are considered to occur in the baby.
The procedure is performed under local anaesthetic which numbs the area of skin prior to inserting the needle. The needle is inserted through the skin of the abdomen and is observed at all times by ultrasound. The needle does not enter the amniotic sac or go near the baby.
In some situations a CVS is performed through the cervix. Local anaesthetic is not required as the instrument passes through the cervical canal. A speculum is used, similar to having a pap smear, so that the cervix can be visualised. The instrument is observed at all times by ultrasound.
Risks associated with CVS
- The risk of pregnancy loss due to a transabdominal CVS is between 0.5 and one per cent (between one in 100 and one in 200 procedures).
- The risk of pregnancy loss due to a transcervical CVS is up to two per cent (one in 50 procedures).
- Light bleeding can occur after a CVS but usually settles without further problems.
- In one per cent of cases, a CVS result may be difficult to interpret due to a situation called mosaicism. This uncertainty can be resolved by performing an amniocentesis.
- In rare cases a result can not be provided from a CVS and repeat sampling may be required.
Amniocentesis is performed after 15 weeks of pregnancy by taking a sample of amniotic fluid that surrounds the baby. Amniotic fluid contains cells from the baby's skin.
The amniocentesis procedure
Amniocentesis is performed by inserting a needle through the skin of the abdomen. The needle is observed at all times by ultrasound. Approximately 20 ml of amniotic fluid is collected.
Risks associated with amniocentesis
- The risk of pregnancy loss due amniocentesis is between 0.5 and one per cent (between one in 100 and one in 200 procedures).
- Infection following amniocentesis is very rare and occurs in less than one in 1000 procedures performed.
- In rare cases a result can not be provided from amniocentesis and repeat sampling may be required.
Care following CVS or amniocentesis
Some women have mild cramping and a small amount of vaginal loss, either bleeding and/or amniotic fluid, after the procedure. This usually resolves without treatment over 24 to 48 hours. If it persists or is heavy, please seek assistance. We recommend women avoid strenuous activities and sexual intercourse for 48 hours following CVS or amniocentesis. Paracetamol is safe to take in pregnancy and may be required following the procedure.
Women with a negative blood group require an injection of Anti-D following CVS or amniocentesis.
CVS and amniocentesis results
A short term result is usually available after two working days. This result provides information about common chromosomal abnormalities including Down syndrome.
The final result which is the most accurate, looks at all the chromosomes and is usually available after two to three weeks.
In rare cases the short term result may differ to the final result and further counselling regarding this discrepancy will be provided.
The sex of the baby can be confirmed by the chromosome result, however this information will only be provided at the woman's request.
For all results further counselling is available.
It is important to note that not all conditions can be detected by CVS or amniocentesis. A normal result can not guarantee that a baby will not have any abnormalities.
Following a CVS or amniocentesis, information will be provided regarding how to obtain the results of these tests.
For further information, we recommend that you speak to one of our staff in Maternal Fetal Medicine or alternatively contact your referring doctor.
It is our aim that women feel fully informed before proceeding with CVS or amniocentesis.
Mater Centre for Maternal Fetal Medicine
Level 7, Mater Mothers' Hospital
Raymond Terrace, South Brisbane Qld 4101
Phone: 07 3163 1896 Fax: 07 3163 1890
Royal College of Obstetricians and Gynecologists. Chorionic Villus sampling (CVS): What you need to know. 2006.
Royal College of Obstetricians and Gynaecologists Amniocentesis: What you need to know. 2006.
Mater acknowledges consumer consultation in the development of this patient information.
Last modified 09/11/2015.