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Eestimated that around 5% of the pregnant population (approximately 30,000 women per annum in the UK) are offered a choice of invasive prenatal diagnostic tests, most commonly amniocentesis or chorionic villus sampling (CVS).
CVS is usually performed between 11 (11+0 ) and 13 (13+6 ) weeks of gestation and involves aspiration or biopsy of placental villi. CVS can be performed using either a transabdominal or a transcervical approach.
Most amniocenteses are performed to obtain amniotic fluid for karyotyping from 15 weeks (15+0 ) onwards. Amniocentesis performed before 15 completed weeks of gestation is referred to as ‘early amniocentesis’.
The CVS test involves taking a 10-50 mg tissue sample from the chorion. If the sample is required after the 12th week, or if transcervical access is impeded, a transabdominal approach may be taken.
The same genetic analyses can be performed on this material as on amniocentesis material, the advantage being earlier results and thus reduced maternal stress if the result indicates a possible therapeutic abortion. Results take about three weeks.
Public Health England information states that:
A RCOG publication notes that (2):