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Chorionic villous sampling (CVS) is usually carried out between 11 and 14 weeks of gestation (but can be carried out up until full term in selected cases), and involves either transabdominal or transcervical aspiration or biopsy of placental villi.
Previously, only amniocentesis was available for accurate karyotype diagnosis. This was carried out between 15–16 weeks. CVS has moved the time of diagnosis earlier. The indications for CVS are:
Amniocentesis can also provide information about a number of chromosomal and non-chromosomal abnormalities, and this information might be complementary to CVS.
Amniocentesis is carried out a few weeks later than CVS (see Amniocentesis).
CVS can be carried out earlier than amniocentesis, but carries a higher risk of miscarriage than other tests that screen for chromosomal or genetic abnormalities (no risk for nuchal translucency plus serum screening, 1:300 for amniocentesis and 1:100 for CVS).
In addition, the combined nuchal translucency and serum marker screening used in the first trimester is not a definitive test, but a statistical evaluation of the likelihood of one of three chromosomal abnormalities: Down syndrome, trisomy 13 and trisomy 18. It cannot predict the likelihood of genetically inherited disorders.
If the patient complains of ongoing pains, bleeding, fluid leak or has signs of chorioamnionitis, then she should be referred back to the specialist who carried out the test for further clinical and ultrasound review to exclude any of the above conditions.
Amniocentesis can also provide information about a number of chromosomal and non-chromosomal abnormalities and this information may be complementary to CVS. Amniocentesis is performed a few weeks later than CVS. Amniocentesis is not used to assist diagnosis of suspected genetic abnormalities.
CVS gives us more information than amniocentesis but carries a higher risk of miscarriage than other tests that screen for chromosomal or genetic abnormalities (no risk for nuchal translucency plus serum screening, 1:300 for amniocentesis, and 1:100 for CVS.
In addition, the combined nuchal translucency and serum marker screening used in the first trimester is not a definitive test but a statistical evaluation of the likelihood of one of three chromosomal abnormalities: Down syndrome, trisomy 13, and trisomy 18. It cannot predict the likelihood of genetically inherited disorders.
NSW Health Centre for Genetics Education: Fact Sheet 26
Women’s and Children’s Health Network
Genetics in Family Medicine: The Australian Handbook for General Practitioners
Page last modified on 30/9/2016.
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