Amniocentesis is very similar to a CVS in that it is a diagnostic test for chromosomal anomalies in the baby including Down's Syndrome. However, unlike a CVS the sample that is taken is a sample of amniotic fluid from around the baby rather than placental tissue. The amniotic fluid contains cells from the baby.
It is performed in a similar way to a CVS, again the needle is passed through the mothers abdomen whilst monitoring the baby and the needle carefully using ultrasound scanning. A small amount of fluid is removed and sent to the laboratory. The procedure takes 1-2 minutes and the fetal heartbeat is checked after the procedure. Because of the relative amounts of fluid around the baby in early pregnancy, an amniocentesis can only safely be performed after 15 weeks.
There may be some abdominal discomfort or leakage of small amounts of fluid vaginally for the first couple of days, which again is common and in the vast majority of cases the pregnancy continues without any problems. You may find it helpful to take simple painkillers like paracetamol. Like a CVS an amniocentesis has a risk of miscarriage of 0.5%. Therefore, if there is a lot of pain or bleeding or if you develop a temperature please seek medical advice.
The results of the Amniocentesis for Down's Syndrome and the other common major chromosomal defects are usually available within 3 working days. A full result of all the chromosomal anomalies including the more rare anomalies would be available 2 weeks after the procedure.
Like CVS very rarely (1:300), the 3 day test fails and you will need to wait for the 2 weeks to get a result. However, unlike 250 CVS results being inconclusive, aminio results are conclusive..