Amniocentesis should be performed after 15 weeks when the uterus is an abdominal organ and the proportion of fluid needed to be removed (15-10 ml) is relatively small compared to the overall liquor volume at this gestation (150-250 ml).
The procedure is performed under aseptic conditions under continuous ultrasound guidance. Best practice is for the operator to introduce a gauge 22-20 needle percu-taneously while he or she is continuously scanning using the free hand. The needle is preferably introduced into a cord free pool of liquor avoiding the placenta. Once in place the inner stylet of the needle is withdrawn and an initial 2 ml of amniotic fluid is withdrawn by an assistant and discarded to avoid maternal contamination. Then a further 15-20 ml is removed using a 10-20 ml syringe. A few operators use a needle guide attached to the transducer, but this has the disadvantage of being less flexible if the needle needs to be realigned.
There is clearly a learning curve with any invasive procedure. Studies have demonstrated the significance of operator experience in terms of both failed attempts and miscarriage rates. Earlier studies had suggested that the difference in miscarriage rates in operators performing over 50 cases per year (0.3%) was considerably lower than ones performing less than 10 cases per year (3.7%) . Amniocentesis is therefore not a routine procedure and it is recommended by the Royal College of Obstetricians and Gynaecologists (RGOG) that it is only performed by adequately trained individuals with at least 50-100 supervised procedures and 50 procedures per annum to maintain their skills. In general only two needle insertions should be attempted and if these fail then the woman should be referred to a tertiary level fetal medicine unit for repeat attempts.
The miscarriage rate for amniocentesis is generally quoted as 1:100 (1%) and is based on the single randomized controlled trial of second trimester amniocentesis by Tabor et al.  in Denmark in 1986. He demonstrated that the women randomized to not have amniocentesis had a miscarriage rate of 0.7% compared to 1.7% in the group who had an amniocentesis and therefore suggested that amniocentesis increased the background miscarriage rate by 1%. More recent uncontrolled series suggest lower miscarriage rates of 0.5%, but it is important that each tertiary level fetal medicine unit should audit their own amniocen-tesis and chorionic villus sampling (CVS) outcomes and be able to quote an individual miscarriage rate for the unit.
Leakage of amniotic fluid vaginally following an amnio-centesis is relatively common occurring in up to 2% but is almost always self-limiting and associated with a normal outcome. Post-amniocentesis chorioamnionitis is also rare occurring in <1.5/1000. However, if signs of chorioamnionitis should be apparent following a recent amniocentesis a repeat amniocentesis with gram staining and culture of the amniotic fluid should be undertaken. If infection is confirmed immediate emptying of the uterine cavity is needed to prevent maternal septicaemia.
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