Introduction
Once a woman has been given a high risk of aneuploidy based on a Down screening, a structural anomaly detected on scan or because of her previous history an obstetrician would normally counsel the women on the options of invasive karyotyping.
This counselling should be based on the risk of aneuploidy, the voluntary nature of the test, the option of no testing, the technique of the proposed test, the procedure-related loss rate and other common complications associated with the test, the timing of the result and the possible management options depending on the result of the test. This decision to balance the potential risk of the loss of an unaffected fetus against that of having an affected child is a very difficult and traumatic one and it is
important that the parents are not rushed into a premature decision.
1. Amniocentesis
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).
Procedure: The procedure is performed under aseptic conditions under continuous ultrasound guidance. Best practice is for the operator to introduce a gauge 22–20 needle percutaneously 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 Once in place the inner stylet of the needle is withdrawn and an initial 2 ml of amniotic fluid is withdrawn by an assistant and discarded to avoid maternal contamination.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. 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%) .